·                  the difficulties in obtaining and maintaining regulatory approval of our product candidates, and the labeling under any approval we may obtain;

·                  our plans and ability to develop, manufacture and commercialize our product candidates;

·                  our failure to recruit or retain key scientific or management personnel or to retain our executive officers;

·                  the size and growth of the potential markets for our product candidates and our ability to serve those markets;

·                  regulatory developments in the United States and foreign countries;

·                  the rate and degree of market acceptance of any of our product candidates;

·                  obtaining and maintaining intellectual property protection for our product candidates and our proprietary technology;

·                  the successful development of our commercialization capabilities, including sales and marketing capabilities;

·                  recently enacted and future legislation and regulation regarding the healthcare system;

·                  the success of competing therapies and products that are or become available;

·                  our dependence on collaboration agreements with other pharmaceutical companies, such as Baxalta and SymBio, for commercializationability to maintain the listing of our productscommon stock on a national securities exchange;

·                  the potential for third party disputes and our ability to achieve certain milestones under those agreements; andlitigation;

·                  the performance of third parties, including contract research organizations or CROs(“CROs”) and third-party manufacturers.manufacturers; and

·                  our expectations regarding CRO transition.

Any forward-looking statements that we make in this prospectus and the documents incorporated by reference herein speak only as of the date of such statement, and we undertake no obligation to update such statements to reflectwhether as a result of any new information, future events, changed circumstances or circumstances after the date of this prospectus or to reflect the occurrence of unanticipated events.otherwise. Comparisons of results for current and any prior periods are not intended to express any future trends or indications of future performance, unless expressed as such, and should only be viewed as historical data.

You should also read carefully the factors described in the “Risk Factors” section of this prospectus and in our annual report on Form 10-K filed with the SEC on March 30, 2015,documents incorporated by reference herein, to better understand significantthe risks and uncertainties inherent in our business and underlying any forward-looking statements. As a result of these factors, actual results could differ materially and adversely from those anticipated or implied inwe cannot assure you that the forward-looking statements in this reportprospectus and in documents incorporated by reference herein will prove to be accurate. Furthermore, if our forward-looking statements prove to be inaccurate, the inaccuracy may be material. In light of the significant uncertainties in these forward-looking statements, you should not place undue reliance onregard these statements as a representation or warranty by us or any forward-looking statements.other person that we will achieve our objectives and plans in any specified timeframe, or at all.

THE LINCOLN PARK TRANSACTION

General

On October 8, 2015, we entered intoWe obtained the Purchase Agreementindustry, market and the Registration Rights Agreement with Lincoln Park. Pursuant to the terms of the Purchase Agreement, Lincoln Park made the Initial Purchase of 846,755 shares of our common stock for a total purchase price of $1,500,000 and has agreed to purchase from us up to an aggregate of $15,000,000 of our common stock (subject to certain limitations) from time to time over a 36-month period. Pursuant to the terms of the Registration Rights Agreement, we have filed with the SEC the registration statement that includescompetitive position data in this prospectus to register for resale under the Securities Act the shares that have been or may be issued to Lincoln Park under the Purchase Agreement.

Concurrently with the execution of the Purchase Agreement on October 8, 2015, Lincoln Park purchased 846,755 shares ofand in documents incorporated by reference herein from our common stock for total proceeds of $1,500,000, the Initial Purchase,own internal estimates and we issued to Lincoln Park 200,000 shares of our common stockresearch as a fee for its commitment to purchase shares of our common stock under the Purchase Agreement.  We do not have the right to commence any further sales to Lincoln Park under the Purchase Agreement until the SEC has declared effective the registration statement of which this prospectus forms a part. Thereafterwell as from industry and upon satisfaction of the other conditions set forth in the Purchase Agreement, we may, from time to timegeneral publications and at our sole discretion, direct Lincoln Park to purchase shares of our common stock in amounts up to 100,000 shares on any single business day, which amounts may be increased but in no event greater than $1,000,000 per such purchase.  The purchase price per share is based on the market price of our common stock immediately preceding the time of sale as computed under the Purchase Agreement without any fixed discount.    Lincoln Park may not assign or transfer its rightsresearch surveys and obligations under the Purchase Agreement.

Purchase of Shares Under the Purchase Agreement

Under the Purchase Agreement, on any business day selectedstudies conducted by us, we may direct Lincoln Park to purchase up to 100,000 shares of our common stock on any such business day.  On any daythird parties. Industry publications and surveys generally state that the closing sale price of our common stockinformation contained therein has been obtained from sources believed to be reliable. We believe this data is not below $1.50 the purchase amount may be increased, at our sole discretion, to up to 125,000 shares of our common stock, on any day that the closing sale price of our common stock is not below $2.00 the purchase amount may be increased, at our sole discretion, to up to 150,000 shares of our common stock, on any day that the closing sale price of our common stock is not below $2.50 the purchase amount may be increased, at our sole discretion, to up to 175,000 shares of our common stock, on any day that the closing sale price of our common stock is not below $3.00 the purchase amount may be increased, at our sole discretion, to up to 200,000 shares of our common stock and on any day that the closing sale price of our common stock is not below $3.50, the purchase amount may be increased, at our sole discretion, to up to 250,000 shares of our common stock.  The amount of any single Regular Purchase may not exceed $1,000,000 per purchase.     The purchase price per share for each such Regular Purchase will be equal to the lower of:

·                  the lowest sale price for our common stock on the purchase date of such shares; or

·                  the arithmetic average of the three lowest closing sale prices for our common stock during the 10 consecutive business days ending on the business day immediately preceding the purchase date of such shares.

In addition to Regular Purchases described above, we may also direct Lincoln Park, on any business day on which we have properly submitted a Regular Purchase notice and our Closing Price is not below $0.50, to purchase an additional amount of our common stock, which we refer to as an Accelerated Purchase, not to exceed the lesser of:

·                  30% of the aggregate shares of our common stock traded during normal trading hours on the purchase date; and

·                  three times the number of purchase shares purchased pursuant to the corresponding Regular Purchase.

The purchase price per share for each such Accelerated Purchase will be equal to the lower of:

·                  97% of the volume weighted average price during (i) the entire trading day on the purchase date, if the volume of shares of our common stock traded on the purchase date has not exceeded a volume maximum calculatedaccurate in accordance with the Purchase Agreement, or (ii) the portion of the trading day of the purchase date (calculated starting at the beginning of normal trading hours) until such time at which the volume of shares of our common stock traded has exceeded such volume maximum; or

·                  the closing sale price of our common stock on the accelerated purchase date.

In the case of both Regular Purchases and Accelerated Purchases, the purchase price per share will be equitably adjusted for any reorganization, recapitalization, non-cash dividend, stock split, reverse stock split or other similar transaction occurring during the business days used to compute the purchase price.

Other than as set forth above, there are no trading volume requirements or restrictions under the Purchase Agreement, and we will control the timing and amount of any sales of our common stock to Lincoln Park.

Events of Default

Events of default under the Purchase Agreement include the following:

·                  the effectiveness of the registration statement of which this prospectus forms a part lapses for any reason (including, without limitation, the issuance of a stop order), or any required prospectus supplement and accompanying prospectus are unavailable for the resale by Lincoln Park of our common stock offered hereby, and such lapse or unavailability continues for a period of 10 consecutive business days or for more than an aggregate of 30 business days in any 365-day period;

·                  suspension by our principal market of our common stock from trading for a period of three consecutive business days;

·                  the de-listing of our common stock from our principal market, provided our common stock is not immediately thereafter trading on the New York Stock Exchange, the NASDAQ Global Select Market, the NASDAQ Global Market, the NYSE Amex or the OTC Bulletin Board (or nationally recognized successor thereto);

·                  the transfer agent’s failure for five business days to issue to Lincoln Park shares of our common stock which Lincoln Park is entitled to receive under the Purchase Agreement;

·                  any breach of the representations or warranties or covenants contained in the Purchase Agreement or any related agreement which has or which could have aall material adverse effect on us subject to a cure period of five business days;

·                  if we issue more than 19.99% of the Company’s aggregate outstanding Common Stock, determinedrespects as of the date of the Purchase Agreement, in violation of the Nasdaq Global Select Market rules;

·                  any voluntary or involuntary participation or threatened participation in insolvency or bankruptcy proceedings by or against us; or

·                  if at any time we are not eligible to transfer our common stock electronically or a material adverse change in our business, financial condition, operations or prospects has occurred.

Lincoln Park does not have the right to terminate the Purchase Agreement upon any of the events of default set forth above. During an event of default, all of which are outside of Lincoln Park’s control, we cannot initiate any Regular Purchases or Accelerated Purchases under the Purchase Agreement.

Our Termination Rights

We have the unconditional right, at any time, for any reason and without any payment or liability to us, to give notice to Lincoln Park to terminate the Purchase Agreement. In the event of bankruptcy proceedings by or against us, the Purchase Agreement will automatically terminate without action of any party.

No Short-Selling or Hedging by Lincoln Park

Lincoln Park has agreed that neither it nor any of its affiliates shall engage in any direct or indirect short-selling or hedging of our common stock during any time prior to the termination of the Purchase Agreement.

Effect of Performance of the Purchase Agreement on Our Stockholders

All 5,200,000 shares registered in this offering which may be sold by us to Lincoln Park under the Purchase Agreement are expected to be freely tradable. It is anticipated that shares registered in this offering will be sold over a period of up to 36-months commencing on the date that the registration statement including this prospectus becomes effective. The sale by Lincoln Park of a significant amount of shares registered in this offering at any given time could cause the market price of our common stock to decline and to be highly volatile. Lincoln Park may ultimately purchase all, some or none of the remaining 4,153,245 shares of common stock registered in this offering not yet issued. If we sell these shares to Lincoln Park, Lincoln Park may sell all, some or none of such shares. Therefore, sales to Lincoln Park by us under the Purchase Agreement may result in substantial dilution to the interests of other holders of our common stock. In addition, if we sell a substantial number of shares to Lincoln Park under the Purchase Agreement, or if investors expect that we will do so, the actual sales of shares or the mere existence of our arrangement with Lincoln Park may make it more difficult for us to sell equity or equity-related securities in the future at a time and at a price that we might otherwise wish to effect such sales. However, we have the right to control the timing and amount of any sales of our shares to Lincoln Park and the Purchase Agreement may be terminated by us at any time at our discretion without any cost to us.

Pursuant to the terms of the Purchase Agreement, we have the right, but not the obligation, to direct Lincoln Park to purchase up to $16,500,000 of our common stock, inclusive of the 846,755 shares issued to Lincoln Park for the $1,500,000 Initial Purchase and exclusive of the 200,000 shares issued to Lincoln Park as a commitment fee.  Depending on the price per share at which we sell our common stock to Lincoln Park, we may be authorized to issue and sell to Lincoln Park under the Purchase Agreement more shares of our common stock than are offered under this prospectus. If we choose to do so, we must first register for resale under the Securities Act any such additional shares, which could cause additional substantial dilution to our stockholders. The number of shares ultimately offered for resale by Lincoln Park under this prospectus is dependent upon the number of shares we direct Lincoln Park to purchase under the Purchase Agreement.

The following table sets forth the amount of gross proceeds we would receive from Lincoln Park from our sale of shares to Lincoln Park under the Purchase Agreement at varying purchase prices:

(1)         Although the Purchase Agreement provides that we may sell up to $16,500,000 of our common stock to Lincoln Park, we are only registering 5,200,000 shares under this prospectus, which may or may not cover all the shares

we ultimately sell to Lincoln Park under the Purchase Agreement, depending on the purchase price per share. As a result, we have included in this column only those shares that we are registering in this offering.

(2)         The number of registered shares to be issued excludes the 200,000 commitment shares because no proceeds will be attributable to such commitment shares.

(3)         The denominator is based on 23,691,471 shares outstanding as of October 8, 2015 prior to the issuance of shares under the Purchase Agreement, adjusted to include the 846,755 shares issued to Lincoln Park for the $1,500,000 Initial Purchase and the 200,000 shares issued to Lincoln Park as commitment shares in connection with this offering and the number of shares set forth in the adjacent column which we would have sold to Lincoln Park at the applicable assumed average purchase price per share.  The numerator does not include the 200,000 shares issued to Lincoln Park as commitment shares in connection with this offering, and is based on the number of shares registered in this offering to be issued under the Purchase Agreement.  The number of shares in such column does not include shares that may be issued to Lincoln Park under the Purchase Agreement which are not registered in this offering.

(4)         The number of registered shares to be issued assumes that we have received stockholder approval to issue shares above the limits imposed by the NASDAQ Global Select Market.

(5)         The closing sale price of our shares on October 13, 2015.

USE OF PROCEEDS

This prospectus relates to shares of our common stock that may be offered and sold from time to time by Lincoln Park. We will receive no proceeds from the sale of shares of common stock by Lincoln Park in this offering. However, we may receive gross proceeds of up to $16,500,000 under the Purchase Agreement. We estimate that the net proceeds to us from this offering will be approximately $12.1 million, based on an assumed public offering price of $1.58 per share, the last reported sale price of our common stock on the Nasdaq Capital Market on January 26, 2018, and after deducting estimated underwriting discounts and commissions and estimated offering expenses payable by us.  If the underwriter exercises its option to Lincoln Park pursuant topurchase additional shares in full, we estimate that the Purchase Agreementnet proceeds will be up to $16,415,000 overapproximately $14.0 million, based on an approximately 36-month period, assuming that we sellassumed public offering price of $1.58 per share, the full amountlast reported sale price of our common stock that we haveon the right, but notNasdaq Capital Market on January 26, 2018, and after deducting the obligation, to sell to Lincoln Park under that agreementestimated underwriting discounts and othercommissions and estimated fees and expenses. See “Plan of Distribution” elsewhere in this prospectus for more information

offering expenses payable by us. We currently expectintend to use the net proceeds we receive under the Purchase Agreementfrom this offering to fund the development of our clinical and preclinical programs, for other research and development activities and for general corporate purposes, which may include capital expenditures and funding our working capital needs. We expect from time to time to evaluate the acquisition of businesses, products and technologies for which a portion of the net proceeds may be used, although we currently are not planning or negotiating any such transactions.

THE SELLING STOCKHOLDER

This prospectus relates toThe amounts actually expended for each purpose may vary significantly depending upon numerous factors, including the possible resale by the selling stockholder, Lincoln Park, of shares of common stock that have been or may be issued to Lincoln Park pursuant to the Purchase Agreement. We are filing the registration statement of which this prospectus forms a part pursuant to the provisionsamount and timing of the Registration Rights Agreement, which we entered intoproceeds from this offering and progress with Lincoln Parkthe clinical development of our product candidates. Expenditures will also depend upon the establishment of collaborative arrangements with other companies, the availability of additional financing and other factors. Investors will be relying on October 8, 2015 concurrently withthe judgment of our executionmanagement regarding the application of the Purchase Agreement, in which we agreed to provide certain registration rights with respect to sales by Lincoln Parkproceeds of theany sale of shares of our common stock that have been or may be issued to Lincoln Park under the Purchase Agreement.stock.

Lincoln Park, as the selling stockholder, may, from time to time, offer and sell pursuant to this prospectus any or allAs of the shares that we have sold or may sell to Lincoln Park under the Purchase Agreement. The selling stockholder may sell some, all or none of its shares. We do not know how long the selling stockholder will hold the shares before selling them, and we currently have no agreements, arrangements or understandings with the selling stockholder regarding the sale of any of the shares.

The following table presents information regarding the selling stockholder and the shares that it may offer and sell from time to time under this prospectus. The table is prepared based on information supplied to us by the selling stockholder, and reflects its holdings as of October 12, 2015. Neither Lincoln Park nor any of its affiliates has held a position or office, or had any other material relationship, with us or any of our predecessors or affiliates. As used in this prospectus, the term “selling stockholder” includes Lincoln Park and any donees, pledgees, transferees or other successors in interest selling shares received after the date of this prospectus, from Lincoln Park as a gift, pledge or other non-sale related transfer. Beneficial ownership is determined in accordancewe cannot specify with Rule 13d-3(d) promulgated by the SEC under the Exchange Act. The percentage of shares beneficially owned prior to the offering is based on 24,773,554 shares of our common stock actually outstanding as of October 12, 2015.

* Less than 1%

(1)         Josh Scheinfeld and Jonathan Cope, the Managing Members of Lincoln Park Capital, LLC, are deemed to be beneficial owners ofcertainty all of the sharesparticular uses of common stock owned by Lincoln Park Capital Fund, LLC. Messrs. Cope and Scheinfeld have shared voting and investment powerthe proceeds from this offering. Accordingly, we will retain broad discretion over the shares being offered underuse of such proceeds. Pending the prospectus filed withuse of the SECnet proceeds from this offering as described above, we intend to invest the net proceeds in connection with the transactions contemplated under the Purchase Agreement. Lincoln Park Capital, LLC is not a licensed broker dealer or an affiliate of a licensed broker dealer.investment-grade, interest-bearing securities.

(2)         Represents 846,755 shares of our common stock purchased by Lincoln Park upon entering into the Purchase Agreement for total proceeds of $1,500,000 and 200,000 shares of our common stock issued to Lincoln Park on October 8, 2015 as a fee for its commitment to purchase additional shares of our common stock under the Purchase Agreement which shares are covered by the registration statement that includes this prospectus. See the description under the heading “The Lincoln Park Transaction” for more information about the Purchase Agreement.

(3)         Based on 24,773,554 outstanding shares of our common stock as of October 12, 2015, which includes 846,755 shares of our common stock purchased by Lincoln Park upon entering into the Purchase Agreement for total proceeds of $1,500,000 and 200,000 shares of our common stock issued to Lincoln Park on October 8, 2015 as a fee for its commitment to purchase additional shares of our common stock under the Purchase Agreement .  Although we may at our discretion elect to issue to Lincoln Park up to an additional amount of $15,000,000 of

our common stock under the Purchase Agreement, other than the shares describedA $0.50 increase or decrease in the immediately preceding sentence, such shares are not beneficially owned by Lincoln Park and are not included in determining the percentage of shares beneficially owned before this offering.

DIVIDEND POLICY

We have never declared or paid any cash dividends on our capital stock. We currently intend to retain all available funds and any future earnings to support our corporation and finance the growth and development of our business. We do not intend to pay cash dividends on our common stock for the foreseeable future.

MARKET PRICE OF COMMON STOCK

Our common stock began trading on the NASDAQ Global Select Market on July 25, 2013 under the symbol “ONTX.” Prior to that time, there was no public market for our common stock. Shares sold in our initialassumed public offering on July 24, 2013 were priced at $15.00price of $1.58 per share.  The following table sets forth for the periods indicated the high and low intra-day sale prices per share, of our common stock as reported on the NASDAQ Global Market:

On October 19, 2015, the last reported sale price of our common stock on the NASDAQ Global SelectNasdaq Capital Market was $1.46 per share.  As of October 12, 2015, we had 169 holders of record of our common stock.  The actualon January 26, 2018, would increase or decrease the net proceeds to us from this offering by $4.0 million, assuming that the number of holdersshares offered by us, as set forth on the cover page of common stock is greater than these numbersthis prospectus, remains the same, after deducting the estimated underwriter discounts and commissions and estimated offering expenses payable by us.

Similarly, each increase or decrease of record holders1,000,000 shares offered by us would increase or decrease the net proceeds to us by approximately $1.5 million, assuming the assumed public offering price of $1.58 per share remains the same, after deducting the underwriting discounts and includes stockholders who are beneficial owners, but whose shares are held in street namecommissions and estimated offering expenses payable by brokers and nominees.  The number of holders of record also does not include stockholders whose shares may be held in trust by other entities.us.

PLAN OF DISTRIBUTIONCAPITALIZATION

The common stock offered by this prospectus is being offered by the selling stockholder, Lincoln Park. The common stock may be sold or distributed from time to time by the selling stockholder directly to one or more purchasers or through brokers, dealers, or underwriters who may act solelyfollowing table presents our cash, cash equivalents and capitalization, as agents at market prices prevailing at the time of sale, at prices related to the prevailing market prices, at negotiated prices, or at fixed prices, which may be changed. The sale of the common stock offered by this prospectus could be effected in one or more of the following methods:September 30, 2017:

·      ordinary brokers’ transactions;on an actual basis; and

·      transactions involving cross or block trades;on an as adjusted basis to give further effect to the sale of 8,650,000 shares of our common stock in this offering at an assumed public offering price of $1.58 per share, the last reported sale price of our common stock on the Nasdaq Capital Market, and after deducting estimated underwriting discounts and commissions and estimate offering expenses payable by us.

You should read this information in conjunction with our consolidated financial statements and notes thereto incorporated by reference into this prospectus.

*

The above table is based on 9,851,164 shares of our common stock outstanding as of September 30, 2017 and exclude:

·                  through brokers, dealers, or underwriters who may act solely as agents907,373 shares of common stock issuable upon the exercise of stock options outstanding at September 30, 2017 with a weighted average exercise price of approximately $41.09 per share;

·                  3,294,771 shares of common stock issuable upon the exercise of outstanding warrants at September 30, 2017 with a weighted average exercise price of approximately $5.10 per share;

·                  “at the market” into an existing market42,355 shares of common stock reserved for the common stock;

·                  in other ways not involving market makers or established business markets, including direct sales to purchasers or sales effected through agents;

·                  in privately negotiated transactions; orfuture issuance under our 2013 Equity Compensation Plan at September 30, 2017; and

·                  any combinationadditional shares of common stock that we may issue to Lincoln Park, pursuant to a purchase agreement we entered into on October 8, 2015, which provides that, upon the foregoing.

In orderterms and subject to comply with the securities laws of certain states, if applicable, the shares may be sold only through registered or licensed brokers or dealers. In addition, in certain states, the shares may not be sold unless they have been registered or qualified for sale in the state or an exemption from the state’s registration or qualification requirement is availableconditions and complied with.

limitation set forth therein, Lincoln Park is committed to purchase up to an “underwriter” withinaggregate of an additional $15 million of shares of our common stock over the meaning of Section 2(a)(11)term of the Securities Act.purchase agreement, should we elect to sell shares to Lincoln Park.

Lincoln Park has informedEach $0.50 increase or decrease in the assumed public offering price per share would increase or decrease the amount of cash and cash equivalents and total stockholders’ equity by approximately $4.0 million, assuming the number of shares offered by us, as set forth on the cover page of this prospectus, remains the same and after deducting the estimated underwriting discounts and commissions and estimated offering expenses payable by us. We may also increase or decrease the number of shares offered in this offering. Each increase or decrease of 1,000,000 shares offered by us would increase or decrease the as adjusted amount of cash and cash equivalents and total stockholders’ equity by approximately $1.5 million, assuming that it intendsthe assumed public offering price remains the same, and after deducting the estimated underwriting discounts and commissions and estimated offering expenses payable by us. The as adjusted information discussed above is illustrative only and will be adjusted based on the actual public offering price and other terms of this offering as determined between us and the underwriter at pricing.

DILUTION

If you invest in our common stock in this offering, your interest will be diluted to use an unaffiliated broker-dealer to effectuate all sales, if any,the extent of the difference between the public offering price per share and the net tangible book value per share of our common stock after this offering. As of September 30, 2017, our historical net tangible book value was $(6.08) million, or $(0.62) per share, based on 9,851,164 shares of our common stock outstanding as of September 30, 2017. Our historical net tangible book value per share represents the amount of our total tangible assets reduced by the amount of our total liabilities, divided by the total number of shares of our common stock outstanding as of September 30, 2017. After giving effect to our sale in this offering of  8,650,000 shares of common stock at an assumed public offering price of $1.58 per share, the last reported sale price of our common stock on the Nasdaq Capital Market on January 26, 2018, and after deducting the underwriting discounts and commissions and estimated offering expenses payable by us, our net tangible book value as of September 30, 2017 would have been $6.0 million, or $0.32 per share. This represents an immediate increase of net tangible book value of $0.94 per share to our existing stockholders and an immediate dilution of $1.26 per share to investors purchasing shares in this offering. The following table illustrates this per share dilution.

Each $0.50 increase or decrease in the assumed public offering price of $1.58 per share, the last reported sale price of our common stock on the Nasdaq Capital Market on January 26, 2018, would increase or decrease our as adjusted net tangible book value per share after this offering by approximately $0.22, and the dilution per share to new investors purchasing shares in this offering by $0.28, respectively, assuming the number of shares offered by us, as set forth on the cover page of this prospectus, remains the same and after deducting underwriting discounts and commissions and estimated offering expenses payable by us. We may also increase or decrease the number of shares to be offered in this offering. Each increase or decrease of 1,000,000 shares offered by us would increase or decrease our as adjusted net tangible book value per share by $0.06, and the dilution per share to new investors purchasing shares in this offering by $0.06, assuming that the assumed public offering price remains the same, and after deducting the estimated underwriting discounts and commissions and estimated offering expenses payable by us. The information discussed above is illustrative only and will be adjusted based on the actual public offering price and other terms of this offering as determined between us and the underwriter at pricing.

The above discussion and table are based on 9,851,164 shares of our common stock outstanding as of September 30, 2017 and exclude:

·                  907,373 shares of common stock issuable upon the exercise of stock options outstanding at September 30, 2017 with a weighted average exercise price of approximately $41.09 per share;

·                  3,294,771 shares of common stock issuable upon the exercise of outstanding warrants at September 30, 2017 with a weighted average exercise price of approximately $5.10 per share;

·                  42,355 shares of common stock reserved for future issuance under our 2013 Equity Compensation Plan at September 30, 2017; and

·                  any additional shares of common stock that itwe may purchase from usissue to Lincoln Park, pursuant to a purchase agreement we entered into on October 8, 2015, which provides that, upon the Purchase Agreement. Such sales will be made at pricesterms and at terms then prevailing or at prices relatedsubject to the then current market price. Each such unaffiliated broker-dealer will beconditions and limitation set forth therein, Lincoln Park is committed to purchase up to an underwriter withinaggregate of an additional $15 million of shares of our common stock over the meaning of Section 2(a)(11)term of the Securities Act.purchase agreement, should we elect to sell shares to Lincoln Park has informed usPark.

To the extent that outstanding options or warrants are exercised, you will experience further dilution. In addition, we may choose to raise additional capital due to market conditions or strategic considerations even if we believe we have sufficient funds for our current or future operating plans. To the extent that additional capital is raised through the sale of equity or convertible debt securities, the issuance of these securities could result in further dilution to our stockholders.

EXECUTIVE COMPENSATION

Overview of Executive Compensation

The compensation committee of our board of directors is responsible for overseeing the compensation of all of our executive officers. In this capacity, our compensation committee annually reviews and approves the compensation of our chief executive officer and other executive officers, including such goals and objectives relevant to the executive officers’ compensation that the committee, in its discretion, determines are appropriate, evaluates their performance in light of those goals and objectives, and sets their compensation based on this evaluation.

2017 Summary Compensation Table

The following table sets forth information for the fiscal years ended December 31, 2017 and 2016 concerning compensation of our principal executive officer and the two most highly compensated executive officers during 2017. We refer to these three executive officers as our “named executive officers.”

(1) Represents discretionary annual bonus amounts paid.

(2) The entries in the option awards column reflect the grant date fair value of the awards, as calculated for financial statement reporting purposes in    accordance with Accounting Standards Codification (ASC) No. 718, Compensation—Stock Compensation.  The option values were calculated using the Black-Scholes option pricing model. These amounts do not represent the actual value realized by the named executive officers. See Note 10 of the Notes to Consolidated Financial Statements for the fiscal year ended December 31, 2016 for a discussion of the relevant assumptions used to determine the valuation of our stock options for accounting purposes.

(3) Includes amounts paid for insurance premiums on behalf of the named executive officer and matching funds paid pursuant to our 401(k) Plan.

Employment Agreements

We have entered into employment agreements with each such broker-dealer will receive commissions from Lincoln Park that will not exceed customary brokerage commissions. In complianceof our named executive officers, and the compensation of our named executive officers is determined, in large part, by the terms of those employment agreements. Following are descriptions of the material terms of each named executive officer’s employment agreement.

Ramesh Kumar, Ph.D.

We entered into an employment agreement with Dr. Kumar on July 1, 2015, which supersedes any prior employment agreements. The employment agreement continues indefinitely, unless terminated in accordance with the guidelinesterms of the Financial Industry Regulatory Authority, Inc., or FINRA, the maximum consideration or discount to be received by any FINRA member or independent broker dealer may not exceed 8% of the aggregate amount of the securities offered pursuant to this prospectus.agreement.

Brokers, dealers, underwritersThe employment agreement provided for an initial base salary of $543,375, subject to adjustment upon annual review by our board of directors, and an annual bonus of up to 55% of such base salary, payable upon our achievement of revenue or agents participating inprofit objectives, specific business plan goals or other performance milestones mutually agreed to by Dr. Kumar and our board of directors, provided that Dr. Kumar remain employed by us throughout the distribution of the shares as agentsperformance year. The bonus may receive compensationbe paid in the form of commissions, discounts, or concessions from the selling stockholder and/or purchasers of the commoncash, stock for whom the broker-dealers may act as agent. The compensation paid to a particular broker-dealer may be less than or in excess of customary commissions. Neither we nor Lincoln Park can presently estimate the amount of compensation that any agent will receive.

We know of no existing arrangements between Lincoln Park or any other stockholder, broker, dealer, underwriter or agent relating to the sale or distribution of theoptions, shares offered by this prospectus. At the time a particular offer of shares is made, a prospectus supplement, if required, will be distributed that will set forth the names of any agents, underwriters or dealers and any compensation from the selling stockholder, and any other required information.

We will pay the expenses incident to the registration, offering, and sale of the shares to Lincoln Park. We have agreed to indemnify Lincoln Park and certain other persons against certain liabilities in connection with the offering of shares of common stock offered hereby, including liabilities arising under the Securities Act or, if such indemnity is unavailable, to contribute amounts required to be paid in respect of such liabilities. Lincoln Park has agreed to

indemnify us against liabilities under the Securities Act that may arise from certain written information furnished to us by Lincoln Park specifically for use in this prospectus or, if such indemnity is unavailable, to contribute amounts required to be paid in respect of such liabilities.

Lincoln Park has represented to us that at no time prior to the Purchase Agreement has Lincoln Park or its agents, representatives or affiliates engaged in or effected, in any manner whatsoever, directly or indirectly, any short sale (as such term is defined in Rule 200 of Regulation SHO of the Exchange Act) of our common stock, or any hedging transaction,a combination thereof, at our compensation committee’s discretion. Dr. Kumar may also be entitled to additional compensation in recognition of extraordinary contributions, at the sole discretion of our compensation committee. On February 12, 2016, we entered into a letter agreement with Dr. Kumar pursuant to which establishesDr. Kumar agreed to a net short positionvoluntary reduction in his base salary from $543,375 to $407,531, effective as of January 1, 2016. For purposes of severance and other benefits calculated based upon base salary, however, Dr. Kumar’s base salary was deemed to remain at $543,375. On December 9, 2016, our board of directors approved the termination of the voluntary salary reduction effective January 1, 2017. Pursuant to this approval, on March 27, 2017, we entered into a letter agreement with respectDr. Kumar under which the voluntary salary reduction was terminated effective January 1, 2017.

Dr. Kumar is entitled to participate in all of our employee benefit plans and programs that are made generally available from time to time to our common stock. Lincoln Park agreedexecutive officers and is entitled to vacation benefits. Pursuant to his employment agreement, Dr. Kumar is entitled to term life insurance coverage in a face amount that is not less than his base salary, a reasonable transportation allowance if we relocate our research facility more than 40 miles from its present location, and up to $10,000 annually for educational programs related to the performance of his duties. If Dr. Kumar dies during his employment, we will be entitled to a $1 million death benefit under a “key man” life insurance policy. Dr. Kumar’s employment agreement contains non-solicitation, non-competition, confidentiality and inventions assignment provisions that, among other things, prevent him from competing with us during the term of his employment and for a specified time thereafter.

If Dr. Kumar’s employment is terminated due to his death, disability, by us for “cause” or by Dr. Kumar without “good reason” during the Purchase Agreement, it, its agents, representativesterm of his employment agreement, we will pay to Dr. Kumar or affiliateshis spouse or estate the balance of his accrued and unpaid salary, unreimbursed expenses, and unused accrued vacation time through the termination date.

If Dr. Kumar’s employment is terminated by us without “cause” or by Dr. Kumar for “good reason,” other than during a change in control protection period, Dr. Kumar will not enter into or effect, directly or indirectly,be entitled to receive severance equal to his current base salary and target bonus for the fiscal year during which his employment ceases. If the termination is during a change in control protection period, Dr. Kumar will be entitled to receive severance equal to two times the sum of his current base salary and target bonus for the fiscal year during which his employment ceases, less any severance previously paid. A change in control protection period commences three months prior to and ends twelve months following a change in control. The Company will also reimburse Dr. Kumar for a portion of his medical insurance costs and all of Dr. Kumar’s incentive stock options that are unvested as of the foregoing transactions.date of such termination would fully vest as of the date of termination.

Steven Fruchtman, M.D.

We have advised Lincoln Parkentered into an employment agreement with Dr. Fruchtman on July 1, 2015, which supersedes any prior employment agreements. The employment agreement continues indefinitely, unless terminated in accordance with the terms of the agreement.

The employment agreement provides for an initial base salary of $420,000, subject to adjustment upon annual review, and subject to the compensation committee’s sole discretion, an annual bonus, based on the performance of Dr. Fruchtman and the Company, of up to 40% of such base salary. The bonus may be paid in the form of cash, stock options, shares of our common stock, or a combination thereof, at our compensation committee’s discretion.

Dr. Fruchtman is entitled to participate in all of our employee benefit plans and programs that itare made generally available from time to time to our executive officers and is requiredentitled to complyvacation benefits. Dr. Fruchtman’s employment agreement contains non-solicitation, non-competition, confidentiality and inventions assignment provisions that, among other things, prevent him from competing with Regulation M promulgatedus during the term of his employment and for a specified time thereafter. The Company will reimburse Dr. Fruchtman for reasonable expenses including certain commuting costs to the Company’s offices.

If Dr. Fruchtman’s employment is terminated due to his death, disability, by us for “cause” or by Dr. Fruchtman without “good reason” during the term of his employment agreement, we will pay to Dr. Fruchtman or his spouse or estate the balance of his accrued and unpaid salary, unreimbursed expenses, and unused accrued vacation time through the termination date.

If Dr. Fruchtman’s employment is terminated by us without “cause” or by Dr. Fruchtman for “good reason,” other than during a change in control protection period, Dr. Fruchtman will be entitled to receive severance equal to the sum of his current base salary and target bonus for the fiscal year during which his employment ceases. If the termination is during a change in control protection period, Dr. Fruchtman will be entitled to receive severance equal to the sum of his current base salary and target bonus for the fiscal year during which his employment ceases. A change in control protection period is the twelve months following a change in control. The Company will also reimburse Dr. Fruchtman for a portion of his medical insurance costs and all of Dr. Fruchtman’s incentive stock options that are unvested as of the date of such termination would fully vest as of the date of termination.

Manoj Maniar, Ph.D.

We entered into an employment agreement with Dr. Maniar on July 1, 2015, which supersedes any prior employment agreements. The employment agreement continues indefinitely, unless terminated in accordance with the terms of the agreement.

The employment agreement provides for an initial base salary of $371,135, subject to adjustment upon annual review by our board of directors, and subject to the compensation committee’s sole discretion, an annual bonus, based on the performance of Dr. Maniar and the Company, of up to 40% of such base salary. The bonus may be paid in the form of cash, stock options, shares of our common stock, or a combination thereof, at our compensation committee’s discretion.

Dr. Maniar is entitled to participate in all of our employee benefit plans and programs that are made generally available from time to time to our executive officers and is entitled to vacation benefits. Dr. Maniar’s employment agreement contains non-solicitation, non-competition, confidentiality and inventions assignment provisions that, among other things, prevent him from competing with us during the term of his employment and for a specified time thereafter.

If Dr. Maniar’s employment is terminated due to his death, disability, by us for “cause” or by

Dr. Maniar without “good reason” during the term of his employment agreement, we will pay to Dr. Maniar or his spouse or estate the balance of his accrued and unpaid salary, unreimbursed expenses, and unused accrued vacation time through the termination date.

If Dr. Maniar’s employment is terminated by us without “cause” or by Dr. Maniar for “good reason,” other than during a change in control protection period, Dr. Maniar will be entitled to receive severance equal to nine-twelfths of the sum of his current base salary and target bonus for the fiscal year during which his employment ceases. If the termination is during a change in control protection period, Dr. Maniar will be entitled to receive severance equal to the sum of his current base salary and target bonus for the fiscal year during which his employment ceases. A change in control protection period is the twelve months following a change in control. The Company will also reimburse Dr. Maniar for a portion of his medical insurance costs and all of Dr. Maniar’s incentive stock options that are unvested as of the date of such termination would fully vest as of the date of termination.

Stock Option and Other Compensation Plans

We maintain our 2013 Equity Compensation Plan for the purpose of attracting key employees, directors and consultants, inducing them to remain with us and encouraging them to increase their efforts to make our business more successful. The plan provides for awards of stock options, stock appreciation rights, restricted stock, restricted stock units, deferred shares and other equity-based awards.

The following table contains certain information regarding equity awards held by the named executive officers as of December 31, 2017:

Outstanding Equity Awards at 2017 Fiscal Year-End

(1) Shares vest in equal monthly installments over four years, 1/48th per month. The first shares vest one month after the date of grant.

(2) Shares vest in equal monthly installments over three years, 1/36th per month. The first shares vest one month after the date of grant.

Potential Payments Upon Termination of Employment or Change in Control

As discussed under the Exchange Act. Withcaption “—Employment Agreements” above, we have agreements with our named executive officers pursuant to which they will receive severance payments upon certain exceptions, Regulation M precludestermination events. The information below describes certain compensation that would be available under our existing plans and arrangements if (i) the selling stockholder, any affiliated purchasers,named executive officer was terminated as of December 31, 2017 or (ii) if a Change in Control, as defined herein, occurred on December 31, 2017 and any broker-dealer or other person who participatesthe named executive officer’s employment had been subsequently terminated on the same date.

Acceleration of Equity Awards

Pursuant to the terms of each named executive officer’s option agreements, in the distribution from bidding for or purchasing, or attempting to induce any person to bid for or purchase any security which is the subject of the distribution until the entire distribution is complete. Regulation M also prohibits any bids or purchases made in order to stabilize the priceevent of a security“Change in connectionControl” that occurs during any time prior to such named executive officer’s Termination of Service (as such terms are defined in our 2013 Equity Compensation Plan) with us, all stock options granted pursuant to such option agreement shall fully vest.

Termination Other than for Cause, Death or Disability; Resignation for Good Reason

The payments and benefits to which each named executive officer would be entitled in the distributionevent the named executive officer’s employment is terminated for any reason other than for cause, death, or disability, or if the named executive officer resigns for good reason, whether or not following a “change in control” is described above.

Equity Compensation Plan Information

The following table summarizes the total number of that security.outstanding options and shares available for other future issuances of options under all of our equity compensation plans as of December 31, 2017. All of the foregoing may affect the marketabilityoutstanding awards listed below were granted under our 2013 Equity Compensation Plan. See “Stock Option and Other Compensation Plans—2013 Equity Compensation Plan” above for a summary of the securities offered by this prospectus.2013 Equity Compensation Plan.

This offering will terminateIn accordance with the terms of the 2013 Equity Compensation Plan, on January 1, 2018, the date that allmaximum aggregate number of shares offered by this prospectus have been sold by Lincoln Park.

Ourof our common stock is quoted on the Nasdaq Global Select Marketthat may be issued under the symbol “ONTX”.plan was automatically increased by 200,000 shares, such that immediately after such increase the number of shares remaining available for future issuance under the plan was 257,632.

DESCRIPTION OF CAPITAL STOCK

Our authorized capital stock consists of 75,000,00025,000,000 shares of common stock, par value $0.01 per share, and 5,000,000 shares of preferred stock, par value $0.01 per share. As of October 12, 2015, 24,773,554January 26, 2018, 10,771,163 shares of our common stock, and no shares of our preferred stock, were outstanding.

Common Stock

Subject to the preferences that may be applicable to any outstanding preferred stock, holders of our common stock are entitled to receive ratably any dividends that may be declared by our board of directors out of funds legally available for that purpose. Holders of our common stock are entitled to one vote for each share on all matters voted on by stockholders, including the election of directors. Holders of our common stock do not have any conversion, redemption, sinking fund or preemptive rights. In the event of our dissolution, liquidation or winding up, holders of our common stock are entitled to share ratably in any assets remaining after the satisfaction in full of the prior rights of creditors and the aggregate liquidation preference of any preferred stock then outstanding. The rights, preferences and privileges of the holders of our common stock are subject to, and may be adversely affected by, the rights of the holders of shares of any series of preferred stock that we may designate and issue in the future. All outstanding shares of our common stock are, and any shares of common stock that we may issue in the future will be, fully paid and non-assessable.

Preferred Stock

We may issue any class of preferred stock in any series. Our board of directors has the authority, subject to limitations prescribed under Delaware law, to issue preferred stock in one or more series, to establish from time to time the number of shares to be included in each series and to fix the designation, powers, preferences and rights of the shares of each series and any of its qualifications, limitations and restrictions. Our board of directors can also increase or decrease the number of shares of any series, but not below the number of shares of that series then outstanding. Our board of directors may authorize the issuance of preferred stock with voting or conversion rights that could adversely affect the voting power or other rights of the holders of the common stock. The issuance of preferred stock, while providing flexibility in connection with possible acquisitions and other corporate purposes, could, among other things, have the effect of delaying, deferring or preventing a change in control of our company and may adversely affect the market price of our common stock and the voting and other rights of the holders of common stock.

Delaware Anti-Takeover Law and Provisions in Our Certificate of Incorporation and Bylaws

Delaware Anti-Takeover Law

We are subject to Section 203 of the Delaware General Corporation Law. Section 203 generally prohibits a public Delaware corporation from engaging in a “business combination” with an “interested stockholder” for a period of three years after the date of the transaction in which the person became an interested stockholder, unless:

·      prior to the date of the transaction, the board of directors of the corporation approved either the business combination or the transaction which resulted in the stockholder becoming an interested stockholder;

·      upon consummation of the transaction that resulted in the stockholder becoming an interested stockholder, the interested stockholder owned at least 85% of the voting stock of the corporation outstanding at the time the transaction commenced, excluding specified shares; or

·      at or subsequent to the date of the transaction, the business combination is approved by the board of directors and authorized at an annual or special meeting of stockholders, and not by written consent, by the affirmative vote of at least 662/3%3% of the outstanding voting stock which is not owned by the interested stockholder.

Section 203 defines a “business combination” to include:

·      any merger or consolidation involving the corporation and the interested stockholder;

·      any sale, lease, exchange, mortgage, pledge, transfer or other disposition of 10% or more of the assets of the corporation to or with the interested stockholder;

·      subject to exceptions, any transaction that results in the issuance or transfer by the corporation of any stock of the corporation to the interested stockholder;

·      subject to exceptions, any transaction involving the corporation that has the effect of increasing the proportionate share of the stock of any class or series of the corporation beneficially owned by the interested stockholder; or

·      the receipt by the interested stockholder of the benefit of any loans, advances, guarantees, pledges or other financial benefits provided by or through the corporation.

In general, Section 203 defines an “interested stockholder” as any person that is:

·      the owner of 15% or more of the outstanding voting stock of the corporation;

·      an affiliate or associate of the corporation who was the owner of 15% or more of the outstanding voting stock of the corporation at any time within three years immediately prior to the relevant date; or

·      the affiliates and associates of the above.

Under specific circumstances, Section 203 makes it more difficult for an “interested stockholder” to effect various business combinations with a corporation for a three-year period, although the stockholders may, by adopting an amendment to the corporation’s certificate of incorporation or bylaws, elect not to be governed by this section, effective 12 months after adoption.

Our certificate of incorporation and bylaws do not exclude us from the restrictions of Section 203. We anticipate that the provisions of Section 203 might encourage companies interested in acquiring us to negotiate in advance with our board of directors since the stockholder approval requirement would be avoided if a majority of the directors then in office approve either the business combination or the transaction that resulted in the stockholder becoming an interested stockholder.

Certificate of Incorporation and Bylaws

Provisions of our certificate of incorporation and bylaws may delay or discourage transactions involving an actual or potential change of control or change in our management, including transactions in which stockholders might otherwise receive a premium for their shares, or transactions that our stockholders might otherwise deem to be in their best interests. Therefore, these provisions could adversely affect the price of our common stock. Among other things, our certificate of incorporation and bylaws will:

·      permit our board of directors to issue up to 5,000,000 shares of preferred stock, with any rights, preferences and privileges as they may designate;

·      provide that all vacancies on our board of directors, including as a result of newly created directorships, may, except as otherwise required by law, be filled by the affirmative vote of a majority of directors then in office, even if less than a quorum;

·      require that any action to be taken by our stockholders must be effected at a duly called annual or special meeting of stockholders and not be taken by written consent;

·      provide that stockholders seeking to present proposals before a meeting of stockholders or to nominate candidates for election as directors at a meeting of stockholders must provide advance notice in writing, and also specify requirements as to the form and content of a stockholder’s notice;

·      not provide for cumulative voting rights, thereby allowing the holders of a majority of the shares of common stock entitled to vote in any election of directors to elect all of the directors standing for election; and

·      provide that special meetings of our stockholders may be called only by the board of directors or by such person or persons requested by a majority of the board of directors to call such meetings.

Transfer Agent and Registrar

The transfer agent and registrar for our common stock is Wells Fargo Shareowner Services.Bank, N.A.

Listing

Our common stock is listed on the Nasdaq GlobalCapital Market under the symbol “ONTX.”

UNDERWRITING

LEGAL MATTERSWe have entered into an underwriting agreement dated                    , 2018, with H.C. Wainwright & Co., LLC as the sole book-running manager of this offering. Subject to the terms and conditions of the underwriting agreement, we have agreed to sell to the underwriter and the underwriter has agreed to purchase from us, at the public offering price less the underwriting discounts and commissions set forth on the cover page of this prospectus. The public offering price shown on the cover page of this prospectus was determined by negotiation between us and the underwriter at the time of pricing, and may be at a discount to the current market price.

A copy of the underwriting agreement has been filed as an exhibit to the registration statement of which this prospectus is a part. The shares we are offering are being offered by the underwriter subject to certain conditions specified in the underwriting agreement.

We have been advised by the underwriter that it proposes to offer the shares directly to the public at the public offering price set forth on the cover page of this prospectus. Any shares sold by the underwriter to securities dealers will be sold at the public offering price less a selling concession not in excess of $            per share.

The validityunderwriting agreement provides that the underwriter’s obligation to purchase the shares we are offering is subject to conditions contained in the underwriting agreement. The underwriter is obligated to purchase and pay for all of the shares offered by this prospectus.

No action has been taken by us or the underwriter that would permit a public offering of shares in any jurisdiction where action for that purpose is required. None of the shares included in this offering may be offered or sold, directly or indirectly, nor may this prospectus or any other offering material or advertisements in connection with the offer and sales of any of the shares be distributed or published in any jurisdiction, except under circumstances that will result in compliance with the applicable rules and regulations of that jurisdiction. Persons who receive this prospectus are advised to inform themselves about and to observe any restrictions relating to this offering of the shares and the distribution of this prospectus. This prospectus is neither an offer to sell nor a solicitation of any offer to buy the shares in any jurisdiction where that would not be permitted or legal.

The underwriter has advised us that it does not intend to confirm sales to any accounts over which it exercises discretionary authority.

Underwriting Discounts, Commissions and Expenses

We have agreed to pay an underwriter discount equal to 7% of the aggregate gross proceeds raised in this offering.

The following table shows the public offering price, underwriting discounts and commissions and proceeds, before expenses to us. These amounts are shown assuming both no exercise and full exercise of the underwriter’s option to purchase additional securities.

We estimate the total expenses payable by us for this offering to be approximately $1.6 million, which amount includes (i) an assumed underwriting discounts and commissions of $1.0 million ($1.1 million if the underwriter’s option to purchase additional shares is exercised in full) based upon the assumed public offering price of $1.58 per share (the last reported sale price of our common stock on the Nasdaq Capital Market on January 26, 2018), (ii) an assumed management fee in the amount of $137,000 which represents 1.0% of the assumed aggregate offering price, (iii) $50,000 non-accountable expense allowance payable to the underwriter, (iv) reimbursement of the accountable expenses of the underwriter equal to $110,000 (none of which has been paid in advance), including the legal fees of the underwriter being paid by us, and (v) other estimated expenses of approximately $336,000 which include legal, accounting, printing costs and various fees associated with the registration and listing of our shares.

Underwriter Warrants

We have agreed to issue to the underwriter warrants to purchase a number of shares of our common stock equal to 5% of the aggregate number of shares of common stock sold in this offering. The underwriter warrants will have a term of two years from the issuance date (subject to certain conditions) and an exercise price per share equal to 125% of the public offering price for the shares sold in this offering. Pursuant to FINRA Rule 5110(g), the underwriter warrants and any shares issued upon exercise of the underwriter warrants shall not be sold, transferred, assigned, pledged, or hypothecated, or be the subject of any hedging, short sale, derivative, put or call transaction that would result in the effective economic disposition of the securities by any person for a period of 180 days immediately following the date of effectiveness or commencement of sales of this offering, except the transfer of any security: (i) by operation of law or by reason of our reorganization; (ii) to any FINRA member firm participating in the offering and the officers or partners thereof, if all securities so transferred remain subject to the lock-up restriction set forth above for the remainder of the time period; (iii) if the aggregate amount of our securities held by the underwriter or related persons do not exceed 1% of the securities being offered; (iv) that is beneficially owned on a pro rata basis by all equity owners of an investment fund, provided that no participating member manages or otherwise directs investments by the fund and the participating members in the aggregate do not own more than 10% of the equity in the fund; or (v) the exercise or conversion of any security, if all securities remain subject to the lock-up restriction set forth above for the remainder of the time period. Notwithstanding the foregoing, the underwriter has agreed, to the extent we do not have sufficient authorized shares necessary for the issuance of the underwriter warrants (including, without limitation, reserved shares underlying the underwriter warrants), the exercise period of the underwriter warrants will commence on the date we increase our authorized shares of common stock upon stockholder approval, subject to the limitation on exercise set forth in FINRA Rule 5110(f)(G)(i).

Right of First Refusal

We have also granted the underwriter, for a period of 10 months from the closing date of this offering, a right of first refusal to act as sole book-running manager for each and every future public or private equity or debt offering by us or any of our successors or subsidiaries. We have also agreed to a tail fee equal to the cash and warrant compensation in this offering if any investor to which the underwriter introduced us with respect to this offering during the term of its engagement provides us with further capital in a public or private offering or capital raising transaction, with certain exceptions, during the 6-month period following termination of our engagement of the underwriter.

Option to Purchase Additional Securities

We have granted the underwriter the option to purchase up to 1,297,500 additional shares of common stock at a purchase price of $ 1.58 per share, less the underwriting discounts and commissions. The underwriter may exercise its option at any time, and from time to time, within 30 days from the date of this

prospectus. If any additional shares are purchased pursuant to the option to purchase additional shares of common stock, the underwriter will offer these securities on the same terms as those on which the other securities are being offered herebyhereby.

Nasdaq Capital Market Listing

Our stock is currently traded on the Nasdaq Capital Market under the symbol “ONTX.” On January 26, 2018, the last reported sale price of our common stock was $1.58 per share.

Lock-up Agreements

Our officers and directors and certain other legal matters willof our stockholders have agreed with the underwriter to be passed uponsubject to a lock-up period of 90 days following the date of this prospectus. This means that, during the applicable lock-up period, such persons may not offer for us by Pepper Hamilton LLP, Princeton, New Jersey.sale, contract to sell, sell, distribute, grant any option, right or warrant to purchase, pledge, hypothecate or otherwise dispose of, directly or indirectly, any shares of our common stock or any securities convertible into, or exercisable or exchangeable for, shares of our common stock. Certain limited transfers are permitted during the lock-up period if the transferee agrees to these lock-up restrictions. We have also agreed in the underwriting agreement, subject to certain exceptions, to similar lock-up restrictions on the issuance and sale of our securities for 90 days following the closing of this offering. The underwriter may, in its sole discretion and without notice, waive the terms of any of these lock-up agreements.

Stabilization, Short Positions and Penalty Bids

The underwriter may engage in syndicate covering transactions, stabilizing transactions and penalty bids or purchases for the purpose of pegging, fixing or maintaining the price of our common stock:

· Syndicate covering transactions involve purchases of securities in the open market after the distribution has been completed in order to cover syndicate short positions. Such a naked short position would be closed out by buying securities in the open market. A naked short position is more likely to be created if the underwriter is concerned that there could be downward pressure on the price of the securities in the open market after pricing that could adversely affect investors who purchase in the offering.

· Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specific maximum.

· Penalty bids permit the underwriter to reclaim a selling concession from a syndicate member when the securities originally sold by the syndicate member are purchased in a stabilizing or syndicate covering transaction to cover syndicate short positions.

These syndicate covering transactions, stabilizing transactions and penalty bids may have the effect of raising or maintaining the market prices of our securities or preventing or retarding a decline in the market prices of our securities. As a result, the price of our common stock may be higher than the price that might otherwise exist in the open market. Neither we nor the underwriter make any representation or prediction as to the effect that the transactions described above may have on the price of our common stock. These transactions may be effected on the Nasdaq Capital Market, in the over-the-counter market or on any other trading market and, if commenced, may be discontinued at any time.

In connection with this offering, the underwriter also may engage in passive market making transactions in our common stock in accordance with Regulation M during a period before the commencement of offers or sales of shares of our common stock in this offering and extending through the completion of the distribution. In general, a passive market maker must display its bid at a price not in excess of the highest independent bid for that security. However, if all independent bids are lowered below the passive market maker’s bid that bid must then be lowered when specific purchase limits are exceeded. Passive market making may stabilize the market price of the securities at a level above that which might otherwise prevail in the open market and, if commenced, may be discontinued at any time.

Neither we nor the underwriter make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the prices of our securities. In addition, neither we nor the underwriter make any representation that the underwriter will engage in these transactions or that any transactions, once commenced, will not be discontinued without notice.

Indemnification

We have agreed to indemnify the underwriter against certain liabilities, including certain liabilities arising under the Securities Act, or to contribute to payments that the underwriter may be required to make for these liabilities.

Other Relationships

The underwriter and its respective affiliates have engaged in, and may in the future engage in, investment banking and other commercial dealings in the ordinary course of business with us or our affiliates. The underwriter has received, or may in the future receive, customary fees and commissions for these transactions.

EXPERTS

The consolidated financial statements of Onconova Therapeutics, Inc. at December 31, 2016 and 2015, and for the years then ended, appearing in Onconova Therapeutics, Inc.’sour Annual Report (Form 10-K) for the year ended December 31, 20142016 have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their report thereon (which contains an explanatory paragraph describing conditions that raise substantial doubt about the Company’s ability to continue as a going concern as described in Note 1 to the consolidated financial statements) included therein and incorporated herein by reference. Such financial statements are incorporated herein by reference in reliance upon such report given on the authority of such firm as experts in accounting and auditing.

LEGAL MATTERS

The validity of the securities being offered by this prospectus will be passed upon by Morgan, Lewis & Bockius LLP, Philadelphia, Pennsylvania. The underwriter is being represented in connection with this offering by Lowenstein Sandler LLP, New York, New York.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy and information statements and other information with the SEC. Our SEC filings, including the registration statement, are available to the public from the SEC’s website at www.sec.gov. To receive copies of public records not posted to the SEC’s website at prescribed rates, you may complete an online form at www.sec.gov, send a fax to (202) 772-9337 or submit a written request to the SEC, Office of FOIA/PA Operations, 100 F Street, N.E., Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information.

We also make available free of charge on our website, www.onconova.com, all materials that we file electronically with the SEC, including our annual reports on Form 10-K, quarterly reports on Form 10-Q, current reports on Form 8-K, Section 16 reports and amendments to those reports as soon as reasonably practicable after such materials are electronically filed with, or furnished to, the SEC. Information contained on our website or any other website is not incorporated by reference into this prospectus and does not constitute a part of this prospectus.

5,200,000 SharesINCORPORATION BY REFERENCE

The SEC allows us to “incorporate by reference” the information we file with it, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be part of this prospectus, and information that we file later with the SEC will automatically update and supersede this information. We incorporate by reference the documents listed below:

·                  Our Annual Report on Form 10-K for the fiscal year ended December 31, 2016 that we filed with the SEC on March 29, 2017, including the information required by Part III, Items 10 through 14, of Form 10-K, which is incorporated by reference to our definitive proxy statement for our 2017 annual meeting of stockholders filed on April 12, 2017;

·                  Our Quarterly Reports on Form 10-Q for the fiscal quarters ended March 31, 2017, June 30, 2017 and September 30, 2017 that we filed with the SEC on May 15, 2017, August 14, 2017 and November 9, 2017, respectively;

·                  Our Current Reports on Form 8-K filed with the SEC on April 20, 2017, April 24, 2017, May 18, 2017, May 25, 2017, August 18, 2017, November 13, 2017, November 17, 2017, January 17, 2018 and January 30, 2018;

·                  The description of our common stock contained in our registration statement on Form 8-A filed on July 23, 2013 (Registration no. 001-36020) with the SEC, including any amendment or report filed for the purpose of updating such description; and

·                  All documents filed by us with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act on or after the date of this prospectus and before we terminate the offering under this prospectus.

We will provide without charge to each person, including any beneficial owner, to whom this prospectus is delivered, upon his or her written or oral request, a copy of any or all documents referred to above which have been or may be incorporated by reference into this prospectus but not delivered with this prospectus excluding exhibits to those documents unless they are specifically incorporated by reference into those documents. You can request those documents from us, at no cost, by writing or telephoning us at: Onconova Therapeutics, Inc., 375 Pheasant Run, Newtown, Pennsylvania, 18940, (267) 759-3680, Attention: Suzanne Hutchinson.

The most recent information that we file with the SEC automatically updates and supersedes older information. The information contained in any such filing will be deemed to be a part of this prospectus, commencing on the date on which the filing is made.

Information furnished under Items 2.02 or 7.01 (or corresponding information furnished under Item 9.01 or included as an exhibit) in any past or future Current Report on Form 8-K that we file with the SEC, unless otherwise specified in such report, is not incorporated by reference in this prospectus.

Onconova Therapeutics, Inc.

ONCONOVA THERAPEUTICS, INC.

8,650,000 Shares of Common Stock

PROSPECTUS

Sole Book-Running Manager

H.C. Wainwright & Co.

                                  , 20152018

Table of Contents

PART II
INFORMATION NOT REQUIRED IN PROSPECTUSInformation Not Required In Prospectus