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TABLE OF CONTENTS

Table of Contents

As filed with the Securities and Exchange Commission on August 3, 2017.18, 2023.

Registration No. 333-            333-___________

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM S-3
REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

OLD SECOND BANCORP, INC.
(Exact name of registrant as specified in its charter)

37 South River Street
Aurora, Illinois 60507
(630) 892-0202
(Address, including zip code, and telephone number, including area code, of each registrant's principal executive offices)​

James L. Eccher
President and Chief Executive Officer
Old Second Bancorp, Inc.
37 South River Street
Aurora, Illinois 60507
(630) 892-0202
(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copy to:

J. Brennan Ryan
Nelson Mullins Riley & Scarborough LLP
Atlantic Station
201 17th Street NW, Suite 1700
Atlanta, Georgia 30363
Telephone: (404) 322-6218
Facsimile: (404) 322-6050

Approximate date of commencement of proposed sale to the public:public: From time to time after the effective date of this registration statement.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box.o

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box.ýx

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the
following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.o

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.o

If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box.o

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box.o

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company, or an emerging growth company. See the definitions of "large“large accelerated filer," "accelerated” “accelerated filer," "smaller” “smaller reporting company,"company” and "emerging“emerging growth company"company” in Rule 12b-2 of the Exchange Act. (Check one):

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If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a)7(a)(2)(B) of the ExchangeSecurities Act. o

CALCULATION OF REGISTRATION FEE

(1)

Calculated in accordance with Rule 457(o) under the Securities Act of 1933. The proposed maximum offering price per security will be determined from time to time by the registrant in connection with the issuance of the securities registered by this registration statement. The proposed maximum aggregate offering price has been estimated solely for the purpose of calculating the registration fee. The amount registered is not specified as to each class of securities to be registered hereunder pursuant to General Instruction II.D. to Form S-3 under the Securities Act of 1933.

(2)

Each share of common stock includes one preferred stock purchase right.

(3)

Shares of common stock or preferred stock may be issued in primary offerings, upon conversion of debt securities or preferred stock registered hereby or upon the exercise of warrants or subscription rights to purchase preferred stock or common stock.

(4)

The debt securities to be offered hereunder will consist of one or more series of senior debt securities or subordinated debt securities, or any thereof, as more fully described herein.

(5)

Warrants exercisable for common stock, preferred stock, depositary shares, debt securities or other securities.

(6)

The depositary shares registered hereunder will be evidenced by depositary receipts issued pursuant to a deposit agreement. If the registrant elects to offer to the public fractional interests in shares of preferred stock, then depositary receipts will be distributed to those persons purchasing the fractional interests and the shares will be issued to the depositary under the deposit agreement.

(7)

Rights evidencing the right to purchase common stock, preferred stock, depositary shares, debt securities or other securities.

(8)

$3,477 of the filings fee was previously paid in connection with the filing of the registrant's previous Registration Statement on Form S-3 (SEC File No. 333-214459), filed on November 4, 2016, and relates to securities that remain unsold thereunder. The registrant is applying the unused fees previously paid to offset the registration fee in respect of the securities registered hereunder pursuant to Rule 457(p) under the Securities Act of 1933. Accordingly, of the total filing fee of $17,385 concurrently due hereunder, the net amount of $13,908 is being paid herewith.

The registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until this registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

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TABLE OF CONTENTS

Prospectus

About this Prospectus‌ Prospectus
1

Special Note Regarding Forward-Looking Statements

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Table of Contents2

PROSPECTUS SUMMARY‌
ABOUT THIS PROSPECTUS
4

Risk Factors‌6

Use of Proceeds‌6

Description of Securities We May Offer‌6

Description of Capital Stock‌7

Description of Debt Securities‌11

Description of Warrants‌19

Description of Depositary Shares‌20

Description of Subscription Rights‌22

Description of Stock Purchase Contracts and Stock Purchase Units‌23

Description of Units‌24

Plan of Distribution‌24

Legal Matters‌26

Experts‌26

Where You Can Find More Information‌26

Incorporation of Certain Information By Reference‌26

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About this Prospectus

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission (the "SEC"“SEC”) using a "shelf"“shelf” registration process. Under this shelf registration process, we may sell from time to time, in one or more offerings, on a continuous or delayed basis, any combination of the securities described in this prospectus.

This prospectus only provides you with a general description of the securities we may offer. Each time we offer and sell securities, we will provide a supplement to this prospectus (which may be a pricing supplement, index supplement, product supplement, free writing prospectus and/or other type of offering document or prospectus supplement, and which are together referred to herein as a “prospectus supplement” or a “supplement”) that will contain specific information about the terms of thatthe securities and the offering. A prospectus supplement may include a discussion of any risk factors or other special considerations applicable to those securities or to us. Such prospectus supplement may also add, update or change information contained or incorporated by reference in this prospectus. IfAny information contained in this prospectus will be deemed to be modified or superseded by any inconsistent information contained in a prospectus supplement. Accordingly, if there is any inconsistency between the information in this prospectus and the applicable prospectus supplement, you should rely on the information in the prospectus supplement. You should read this prospectus and the applicable prospectus supplement and any documents that we incorporate by reference in this prospectus or any prospectus supplement, together with the additional information referred to under the heading "Where“Where You Can Find More Information,"” before you invest.

You should rely only on the information contained or incorporated by reference in this prospectus or in any prospectus supplement. We have not authorized anyone to provide you with different or additional information. We are not making an offer to sell or soliciting an offer to buy these securities in any jurisdiction in which the offer or solicitation is not authorized or in which the person making the offer or solicitation is not qualified to do so or to anyone to whom it is unlawful to make the offer or solicitation. You should assume that the information contained or incorporated by reference in this prospectus or any prospectus supplement is accurate only as of the date of that document.

Any of the securities described in this prospectus and in a prospectus supplement may be convertible or exchangeable into, or exercisable for, other securities that are described in this prospectus or will be described in a prospectus supplement, and may be issued separately, together or as part of a unit consisting of two or more securities, which may or may not be separate from one another. The securities offered hereby may include new or hybrid securities developed in the future that combine features of any of the securities described in this prospectus.

The registration statement that contains this prospectus, including the exhibits to the registration statement, also contains additional information about us and the securities offered under this prospectus. You can find the registration statement at the SEC'sSEC’s website atwww.sec.gov or at the SEC office mentioned under the heading "Where You Can Find More Information.".

        In this prospectus, the terms "Company," "we," "us," and "our" refer to Old Second Bancorp, Inc. ReferencesUnless otherwise indicated, currency amounts in this prospectus to the "Bank" mean Old Second National Bank.and in any applicable prospectus supplement are stated in United States dollars.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS
Special Note Regarding Forward-Looking Statements

This prospectus, any related prospectus supplement, and the documents we incorporate by reference in this prospectus and any related prospectus supplement may contain forward-looking statements within the meaning of the federal securities laws, including with respect to management'smanagement’s current beliefs and expectations regarding future plans, strategies and financial performance, regulatory developments, industry and economic trends, and other matters.  Forward-looking statements are identifiable by the inclusion of such qualifications as "expects," "intends," "believes," "may," "will," "would," "could," "should," "plan," "anticipate," "estimate," "possible," "likely"“expects,” “intends,” “believes,” “may,” “will,” “would,” “could,” “should,” “plan,” “anticipate,” “estimate,” “possible,” “likely” or other indications

These and other risks associated with changesand uncertainties are more fully discussed in regulationthe “Risk Factors” and the current regulatory environment, including the Dodd-Frank Wall Street Reform and Consumer Protection Act;

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PROSPECTUS SUMMARY

This summary provides a general description of newthe securities we may offer. This summary is not complete and does not contain all of the information or future events. Our ability to predict results or the actual effect of future plans or strategies is inherently uncertain. In addition, there are other factors that may impact any public company, including ours, which could havebe important to you. For a material adverse effect onmore complete understanding of us and the operations and future prospectsterms of the Companysecurities we may offer, you should read carefully this entire prospectus, including the “Risk Factors” section, the applicable prospectus supplement for the securities and its subsidiaries. These risksthe other documents we refer to and uncertainties should be considered in evaluating forward-looking statementsincorporate by reference. In particular, we incorporate important business and undue reliance should not be placed on such statements.financial information into this prospectus by reference.

OLD SECOND BANCORP, INC.
Old Second Bancorp, Inc.

        The Company wasOld Second Bancorp, Inc. is a corporation organized under the laws of the State of Delaware on September 8, 1981. It is a registeredin 1981 that serves as the bank holding company under the Bank Holding Company Act of 1956. The Company conducts a full service community banking and trust business throughfor its wholly owned subsidiaries, includingwholly-owned subsidiary bank, Old Second National Bank. The Bank which we refer to as the "Bank".

        The Company provides financial services through its 26is a national banking locations that are located primarilyassociation headquartered in Aurora, Illinois, and its surrounding communities and throughout the Chicago metropolitan area. These locations include retail officesthat operates through 48 banking centers located in Cook, DeKalb, DuPage, Kane, Kendall, DeKalb, DuPage, LaSalle Will and CookWill counties in Illinois.

        The Bank's full serviceWe are a full-service banking businesses include the customary consumerbusiness offering a broad range of deposit products, trust and commercial productswealth management services, and lending services, that banking institutions provide including demand, NOW, money market, savings, time deposit and individual retirement deposit accounts; commercial, industrial, consumer and real estate lending, including installment loans, agricultural loans, lines of credit and overdraft checking; safe deposit operations; trust services; wealth management services;operations, and an extensive variety of additional services tailored to the needs of individual customers, such as the acquisition of U.S. Treasury notes and bonds, money orders, cashiers'cashiers’ checks and foreign currency, direct deposit, discount brokerage, debit cards, credit cards, and other special services. The BankOur lending activities include making commercial and consumer loans, primarily on a secured basis. Commercial lending focuses on business, capital, construction, inventory and real estate lending. Installment lending includes direct and indirect loans to consumers and commercial customers.

We also offersoffer a full complement of electronic banking services such as online and mobile banking and corporate cash management products including remote deposit capture, mobile deposit capture, investment sweep accounts, zero balance accounts, automated tax payments, ATM access, telephone banking, lockbox accounts, automated clearing house transactions, account reconciliation, controlled disbursement, detail and general information reporting, foreign and domestic wire transfers, vault services for currency and coin, and checking accounts. Commercial and consumer loans are made to corporations, partnerships and individuals, primarily on a secured basis. Commercial lending focuses on business, capital, construction, inventory and real estate lending. Installment lending includes direct and indirect loans to consumers and commercial customers. Additionally, the Bank provideswe provide a wide range of wealth management, investment, agency, and custodial services for individual, corporate, and not-for-profit clients. These services include the administration of estates and personal trusts, as well as the management of investment accounts for individuals, employee benefit plans, and charitable foundations. The BankWe also originatesoriginate residential mortgages, offering a wide range of mortgage products including conventional, government, and jumbo loans. SecondaryWe also handle secondary marketing of those mortgages is also handled at the Bank.mortgages.

As of June 30, 2023, we had consolidated total assets of $5.88 billion, net loans of $3.96 billion, deposits of $4.72 billion and stockholders’ equity of $514.0 million.

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Our principal executive office is located at 37 South River Street, Aurora, Illinois 60507 and our telephone number is (630) 892-0202.

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Additional information about us is included in our filings with the SEC, which are incorporated by reference into this prospectus. See "Where“Where You Can Find More Information"Information” and "Incorporation“Incorporation of Certain Information by Reference"Reference” in this prospectus.

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The Securities We May Offer

TableUnder the shelf registration statement to which this prospectus is a part, we may sell securities consisting of Contentsone or any combination or combinations of the following securities or any combination of these securities from time to time:

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A prospectus supplement, which we will provide each time we offer securities, will describe the amounts, prices and detailed terms of the securities and may describe risks associated with an investment in the securities in addition to those described in the “Risk Factors” section on page 6 of this prospectus.

We may sell the securities to or through underwriters, dealers or agents or directly to purchasers. Each prospectus supplement will set forth the names of any underwriters, dealers or agents involved in the sale of such securities described in that prospectus supplement and any applicable fee, commission or discount arrangements with them.

Listing

If any securities are to be listed or quoted on a securities exchange or quotation system, the applicable prospectus supplement will so indicate. Our common stock is listed on the NASDAQ Global Select Market under the symbol “OSBC.”

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RISK FACTORS
Risk Factors

An investment in our securities involves certain risks. Before making an investment decision,you invest in any of our securities, in addition to the other information included in, or incorporated by reference into, this prospectus, you should carefully read and consider the risks described below as well as those set forth under the heading "Risk Factors"risk factors contained in our most recent Annual Report on Form 10-K, and each subsequently filed Annual Report on Form 10-K orin our updates to those Risk Factors in our Quarterly ReportReports on Form 10-Q as well as any risk factors set forth in our other filings with the SEC, in each case, thatand Current Reports on Form 8-K, which are incorporated by reference ininto this prospectus or any prospectus supplement.prospectus. See "Incorporation“Incorporation of Certain Information by Reference"Reference” and "Where“Where You Can Find More Information."” Our business, financial condition or results of operations could be materially adversely affected by any of these risks. The trading price of our securities could decline due to any of these risks, and you may lose all or part of your investment. This prospectus and the documents incorporated by reference ininto this prospectus also contain forward-looking statements that involve risks and uncertainties. Our actual results could differ materially from those anticipated in these forward-looking statements as a result of certain factors, including the risks faced by us.

SharesUse of our common stock are subject to dilution, which could cause our common stock price to decline.Proceeds

        We are generally not restricted from issuing additional shares of our common stock up to the number of shares authorized in our Certificate of Incorporation. We may issue additional shares of our common stock (or securities convertible into common stock) in the future for a number of reasons, including to finance our operations and business strategy (including mergers and acquisitions), to adjust our ratio of debt to equity, to address regulatory capital concerns, or to satisfy our obligations upon the exercise of outstanding options or warrants. We may issue equity securities in transactions that generate cash proceeds, transactions that free up regulatory capital but do not immediately generate or preserve substantial amounts of cash, and transactions that generate regulatory or balance sheet capital only and do not generate or preserve cash. If we choose to raise capital by selling shares of our common stock or securities convertible into common stock for any reason, the issuance would have a dilutive effect on the holders of our common stock and could have a material negative effect on the market price of our common stock.

We face a risk of noncompliance with the Bank Secrecy Act and other anti-money laundering statutes and regulations and corresponding enforcement proceedings.

        The federal Bank Secrecy Act, the Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism Act of 2001, or the "PATRIOT Act," and other laws and regulations require financial institutions, among other duties, to institute and maintain effective anti-money laundering programs and to file suspicious activity and currency transaction reports as appropriate. The federal Financial Crimes Enforcement Network, established by the U.S. Treasury Department to administer the Bank Secrecy Act, is authorized to impose significant civil money penalties for violations of those requirements and has recently engaged in coordinated enforcement efforts with the individual federal banking regulators, as well as the U.S. Department of Justice, Drug Enforcement Administration and Internal Revenue Service. There is also increased scrutiny of compliance with the rules enforced by the Office of Foreign Assets Control. Federal and state bank regulators also have begun to focus on compliance with Bank Secrecy Act and anti-money laundering regulations. If our policies, procedures and systems are deemed deficient or the policies, procedures and systems of the financial institutions that we have already acquired or may acquire in the future are deficient, we would be subject to liability, including fines and regulatory actions such as restrictions on our ability to pay dividends and the necessity to obtain regulatory approvals to proceed with certain aspects of our business plan, including our acquisition plans, which would negatively impact our business, financial condition and results of operations. Failure to maintain and implement adequate programs to combat money laundering and terrorist financing could also have serious reputational consequences for us.

DESCRIPTION OF CAPITAL STOCK
Description of Capital Stock

The following description of our capital stock is a summary only and is subject to, and is qualified in its entirety by reference to, applicable provisions of the Delaware General Corporation Law, as amended,DGCL, our Certificate of Incorporation and our Bylaws, and the description of our common stock contained in the Registration Statement on Form S-1 filed with the SEC on January 17, 2014, as amended on February 27, 2014, March 26, 2014 and April 1, 2014, and the description of preferred share purchase rights included in our most recent amendment to our Registration Statement on Form 8-A filed with the SEC on September 4, 2015, which are incorporated by reference into this prospectus. Bylaws. Although we believe this summary covers the material terms and provisions of our capital stock set forth in our Certificate of Incorporation and Bylaws, it may not contain all of the information that is important to you.you.

In this section “Description of Capital Stock,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

Authorized Shares of Capital Stock

We have the authority to issue 60,000,000 shares of common stock, $1.00 par value per share, and 300,000 shares of preferred stock, $1.00 par value per share. As of June 30, 2017, we had 29,627,086 shares of common stock issued and outstanding and no shares of preferred stock issued and outstanding.

Common Stock

Common Stock

        General.General. As of the date of this prospectus, our Certificate of Incorporation provides that we may issue up to 60,000,000 shares of common stock, $1.00 par value per share. As of June 30, 2017,2023, we had 29,627,08644,705,150 shares of common stock issued and outstanding and there44,665,127 shares of our common stock outstanding. There were 574,000764,606 shares of our common stock underlying options issuedrestricted stock unit awards pursuant to our equity incentive plans 443,209and 967,600 shares of our common stock reservedremaining available for future issuancegrant under our equity incentive plans,the Old Second Bancorp, Inc. 2019 Equity Incentive Plan, as amended and 475,000 shares of our common stock reserved for all non-vested restricted stock awards. Additionally, we have reserved 815,339 shares of our common stock for issuance upon exercise of the warrant that was originally

issued to the U.S. Department of the Treasury through the Capital Purchase Program.restated. All outstanding shares of our common stock are fully paid and nonassessable.

        Each outstanding share of common stock includes one preferred stock purchase right issued under our Tax Benefits Plan, as described below. See "—Series A Junior Participating Preferred Stock" and "—Antitakeover Provisions—Tax Benefits Plan."

Our common stock is listed on the NASDAQ Global Select Market under the symbol "OSBC."“OSBC.”

Dividend Rights.Subject to any rights to receive dividends to which the holders of any outstanding preferred stock may be entitled, holders of our common stock are entitled to receive any cash dividends that may be declared by our board of directors. We are subject to various regulatory policiescertain restrictions on dividend payments under the DGCL as well as restrictions under certain banking laws, regulations and requirements relating to the payment of dividends, including requirements to maintain adequate capital above regulatory minimums.policies. Subject to these restrictions, the declaration and payment of future dividends to holders of our common stock will be at the discretion of our board of directors and will depend upon our earnings and financial condition, our capital requirements and those of our subsidiaries, regulatory conditions and considerations and other factors as our board of directors may deem relevant.

Voting Rights. Each outstanding share of our common stock shall entitle the holder thereof to one vote, and, except as otherwise stated or expressed in a resolution or resolutions adopted by our board of directors providing for the issue of any preferred stock or as otherwise provided by law, the exclusive voting power for all purposes shall be vested in the holders of common stock. The holders of our common stock have no cumulative voting rights. When a quorum is present at any meeting, the vote of a majority of the shares present in person or represented by proxy will decide the election of directors, unless the number of director nominees exceeds the number of directors to be elected at such meeting, in which case, the directors will be elected by a plurality vote. When a quorum is present at any meeting, questions brought before the meeting will be decided by the vote of the holders of a majority of the shares present andhaving voting on such matter, whetherpower present in person or represented by proxy, except when the matter requires a different vote under applicable Delaware law, our Certificate of Incorporation or our Bylaws. Our Certificate of Incorporation provides certain provisions that may limit stockholders'stockholders’ ability to effect a change in control as described under the section below entitled "—“—Antitakeover Provisions—Provisions—Certificate of Incorporation; Bylaws."Bylaws.”

Liquidation Rights.In the event of any liquidation, dissolution or winding up of the Company, whether voluntary or involuntary, after payment or provision for payment of the debts and other liabilities of the Company and the preferential amounts to which the holders of any outstanding preferred stock shall be entitled, the holders of our common stock shall be entitled to share ratably in the remaining assets of the Company. The merger or consolidation of the Company into or with any other corporation, or the merger of any other corporation into it, or a sale of all or substantially all of the assets of the Company, or, any purchase or redemption of shares of stock of the Company of any class, shall not be deemed to be a liquidation, dissolution or winding up of the Company.

Preemptive Rights. The holders of our common stock have no preemptive rights.

Miscellaneous.Shares of our common stock are not convertible into shares of any other class of capital stock. The issued and outstanding shares of our common stock are fully paid and nonassessable.

Preferred Stock

Preferred Stock

General.We may issue up to 300,000 shares of preferred stock, $1.00 par value per share, from time to time in one or more series. Our board of directors, without further approval of our stockholders, has the authority to fix the dividend rights and terms, conversion rights, voting rights, redemption rights and terms, liquidation preferences, sinking funds and any other rights, preferences,

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privileges and restrictions applicable to each series of preferred stock. The issuance of preferred stock, while providing flexibility in connection with possible acquisitions and other corporate purposes, could, among other things, adversely affect the voting power of the holders of our common stock.

The authorization of preferred stock could have the effect of impeding the acquisition of control of the Company by means of a tender offer, a proxy fight, open-market purchases or otherwise in a transaction not approved by our board of directors. See “—Antitakeover Provisions—Certificate of Incorporation; Bylaws.”

As of June 30, 2023, we had no shares of preferred stock issued and outstanding.

We will describe the particular terms of any series of preferred stock being offered in the applicable prospectus supplement relating to that series of preferred stock. The following description and any description of preferred stock in a prospectus supplement is only a summary and is subject to and qualified in its entirety by reference to the certificate of amendment to our Certificate of Incorporation relating to the particular series of preferred stock, a copy of which we will file with the SEC in connection with the sale of any series of preferred stock.

The prospectus supplement relating to a particular series of preferred stock will contain a description of the specific terms of that series of preferred stock. Those terms may include:include:

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the number of shares being offered;

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the title, designation and liquidation preference per share;

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the purchase price;

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any voting rights;

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the dividend rate or method for determining that rate;

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the dates on which dividends will be paid;

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whether dividends will be cumulative or noncumulative and, if cumulative, the dates from which dividends will begin to accumulate;

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any applicable redemption or sinking fund provisions;

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any applicable conversion provisions;

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whether we have elected to offer depositary shares with respect to that series of preferred stock;

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the exchange or market, if any, where the preferred stock will be listed or traded; and

the number of shares being offered;

The shares of preferred stock will, when issued against full payment of their purchase price, be fully paid and nonassessable. Except as otherwise may be specified in the prospectus supplement relating to a particular series of preferred stock, holders of preferred stock will not have any preemptive or subscription rights to acquire any class or series of our capital stock.

        The authorization of the preferred stock could have the effect of impeding the acquisition of control of the Company by means of a tender offer, a proxy fight, open-market purchases or otherwise in a transaction not approved by our board of directors. See "—Antitakeover Provisions—Certificate of Incorporation; Bylaws."

Dividend Rights.If you purchase preferred stock being offered by use of this prospectus and an applicable prospectus supplement, you will be entitled to receive, when, as and if declared by our board of directors, dividends at the rates and on the dates set forth in the prospectus supplement. Dividend rates may be fixed, variable or both. The nature, amount, rates, timing and other details of dividend rights for a series of preferred stock will be described in the applicable prospectus supplement and will be payable in preference to, or in such relation to, the dividends payable on any other class or classes or series of our stock, as described in the applicable prospectus supplement. We are subject to various regulatory policies and requirements relating to the payment of dividends, including requirements to maintain adequate capital above regulatory minimums.

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Voting Rights.The voting rights of preferred stock of any series being offered will be described in the applicable prospectus supplement.

Liquidation Rights.In the event that we liquidate, dissolve or wind-up our affairs, either voluntarily or involuntarily, holders of our preferred stock will be entitled to receive liquidating distributions in the amount set forth in the applicable prospectus supplement, plus accrued and unpaid dividends, if any, before we make any distribution of assets to the holders of our common stock or any junior preferred stock. If we fail to pay in full all amounts payable with respect to preferred stock being offered by us and any stock having the same rank as that series of preferred stock, the holders of the preferred stock and of that other stock will share in any distribution of assets in proportion to the full respective preferential amounts to which they are entitled. After the holders of each series of preferred stock and any stock having the same rank as the preferred stock are paid in full, they will have no right or claim to any of our remaining assets. For any series of preferred stock being offered by this prospectus and an applicable prospectus supplement, neither the sale of all or substantially all of our property or business nor a merger or consolidation by us with any other corporation will be considered a dissolution, liquidation or winding-up of our business or affairs.

Redemption.         Redemption.The terms, if any, on which shares of a series of preferred stock being offered may be redeemed will be described in the applicable prospectus supplement. The preferred stock of a series may be redeemed in such amount or amounts, and at such time or times, if any, as may be provided in respect of that particular series of preferred stock. Preferred stock may be redeemed by the Company only to the extent legally permissible.

Conversion Rights.The applicable prospectus supplement will state the terms, if any, on which shares of a series of preferred stock being offered are convertible into shares of our common stock or another series of our preferred stock.

Anti-Takeover Provisions

Series A Junior Participating Preferred Stock

        We have 10,000 shares of preferred stock designated as Series A Junior Participating Preferred Stock, $1.00 par value per share, which we refer to as the Series A Preferred Stock, none of which were issued and outstanding as of June 30, 2017. We declared a dividend of one right for each share of our common stock in conjunction with the adoption of a stockholders' rights and tax benefits plan which is discussed under the heading "—Antitakeover Provisions—Tax Benefits Plan," which represented the right to purchase one one-thousandth of a share of Series A Preferred Stock. Pursuant to the terms of the stockholders' rights and tax benefits plan, one right will be issued with each share of common stock issued by the Company, which right will be governed by the stockholders' rights and tax benefits plan.

Anti-Takeover Provisions

General. Certain provisions of our Certificate of Incorporation, our Bylaws and the Delaware General Corporation Law and our Tax Benefits Plan (as defined below)DGCL may have the effect of impeding the acquisition of control of the Company by means of a tender offer, a proxy fight, open-market purchases or otherwise in a transaction not approved by our board of directors. These provisions may have the effect of discouraging a future takeover attempt which is not approved by our board of directors, but which individual stockholders may deem to be in their best interests or in which our stockholders may receive a substantial premium for their shares over then-current market prices. As a result, stockholders who might desire to participate in such a transaction may not have an opportunity to do so. Such provisions will also render the removal of our current board of directors or management more difficult.

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Certificate of Incorporation; Bylaws. These provisions of our Certificate of Incorporation and our Bylaws include the following:

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Our board of directors may issue additional authorized shares of our capital stock to deter future attempts to gain control of the Company, and has the authority to determine the terms of any one or more series of preferred stock, such as voting rights, conversion rates and liquidation preferences. As a result of the ability to fix voting rights for a series of preferred stock, our board has the power, to the extent consistent with its fiduciary duties, to issue a series of preferred stock to persons friendly to management in order to attempt to block a merger or other transaction by which a third party seeks control, and thereby assist the incumbent board of directors and management to retain their respective positions;



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Our Certificate of Incorporation provides for the division of our board of directors into three classes of directors of approximately equal size, which is referred to as a classified board. Our directors are elected for three-year terms. Consequently only one-third of our directors are up for election in any given year. Our classified board, therefore, may make it more difficult for a shareholder to acquire immediate control of the Company and immediately remove the incumbent management through a proxy contest. Because the terms of only approximately one-third of the incumbent directors expire each year, at least two annual elections are required before a majority of our directors can be replaced. In distinction, if we had a non-classified board, a majority of our directors could theoretically be replaced at one annual meeting;



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Our Certificate of Incorporation does not provide for cumulative voting for any purpose, and our Certificate of Incorporation also provides that any action required or permitted to be taken by stockholders may be taken only at an annual or special meeting and prohibits stockholder action by written consent in lieu of a meeting;



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Certain transactions with interested stockholders (including any merger or consolidation, the sale, lease or exchange of all of substantially all assets, any issuance or transfer of any voting securities to in exchange for cash, assets or securities) must be approved by at least 75% of our outstanding voting stock, unless approved by not less than a majority of our disinterested directors;



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When evaluating a proposal by another person to make a tender or exchange offer for an equity security, to merge or consolidate with us or to purchase or otherwise acquire all or substantially all of our assets, our Certificate of Incorporation allows the board of directors to consider non-stockholder interests, such as the social and economic effects of the transaction on us and our subsidiaries and the other elements of the communities in which we and our subsidiaries operate or are located; and



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An amendment of our Certificate of Incorporation must be approved by a majority vote of the board of directors and also by a majority vote of the outstanding shares of our common stock, provided, however, that an affirmative vote of at least 75% of the outstanding voting stock entitled to vote is required to amend or repeal certain provisions of our Certificate of Incorporation, including provisions (a) concerning directors, (b) governing the purchase by the Company of our voting stock, (c) relating to considerations of the board of directions in evaluating acquisition proposals, (d) limiting business combinations with interested stockholders, (e) limiting our stockholders' ability to act by written consent, and (f) regarding amendment of the foregoing supermajority provisions of our Certificate of Incorporation. Our Bylaws may be amended only by vote of a majority of the board of directors.

        Tax Benefits Plan.    On September 12, 2012, the Company and the Bank entered into an Amended and Restated Rights Agreement and Tax Benefits Preservation Plan, which the parties subsequently amended on April 3, 2014 and September 2, 2015 (as amended, the "Tax Benefits Plan"). The Tax

and provisions of the securities to which any prospectus supplement may relate. The specific terms and provisions of a series of debt securities and the extent to which the general terms and provisions may also apply to a particular series of debt securities will be described in the applicable prospectus supplement.

Each prospectus supplement will specify the particular terms of the securities offered. These provisions may include, among other things and to the extent applicable, the following:following:

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the title of the debt securities, including, as applicable, whether the debt securities will be issued as senior debt securities, senior subordinated debt securities, subordinated debt securities or junior subordinated debt securities, and any subordination provisions particular to the series of debt securities;



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any limit on the aggregate principal amount of the debt securities;



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if other than 100% of the aggregate principal amount, the percentage of the aggregate principal amount at which we will sell the debt securities (i.e., original issuance discount);



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the date or dates, whether fixed or extendable, on which the principal of the debt securities will be payable;



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the rate or rates, which may be fixed or variable, at which the debt securities will bear interest, if any, the date or dates from which any such interest will accrue, the interest payment dates on which we will pay any such interest, the basis upon which interest will be calculated if other than that of a 360-day year consisting of twelve 30-day months, and, in the case of registered securities, the record dates for the determination of holders to whom interest is payable;



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any provisions relating to the issuance of the debt securities of the series at an original issue discount;



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the place or places where the principal of, and any premium or interest on, the debt securities will be payable and, if applicable, where the debt securities may be surrendered for conversion or exchange;

Unless otherwise indicated in the applicable prospectus supplement, we will issue debt securities in fully registered form without coupons and in denominations of $1,000 and in integral multiples of $1,000, and interest will be computed on the basis of a 360-day year of twelve 30-day months. If any interest payment date or the maturity date falls on a day that is not a business day, then the payment will be made on the next business day without additional interest and with the same effect as if it were made on the originally scheduled date.

Unless otherwise indicated in the applicable prospectus supplement, the trustee will act as paying agent and registrar for the debt securities under the indenture. We may also act as paying agent under the indenture.

The applicable prospectus supplement will contain a description of U.S. federal income tax consequences relating to the debt securities, to the extent applicable.

Covenants

The applicable prospectus supplement will describe any covenants, such as restrictive covenants restricting us or any of our subsidiaries from incurring, issuing, assuming or guarantying any indebtedness or restricting us or any of our subsidiaries from paying dividends or acquiring any of our or its capital stock.

Notwithstanding the foregoing, we may transfer all of our property and assets to another entity if, immediately after giving effect to the transfer, such entity is our wholly-owned subsidiary. The term "wholly-owned subsidiary"“wholly-owned subsidiary” means any subsidiary in which we and/or our other wholly-owned subsidiaries own all of the outstanding capital stock.

Modification and Waiver

Unless we indicate otherwise in the applicable prospectus supplement, under the indenture, some of our rights and obligations and some of the rights of the holders of the debt securities may be modified or amended with the consent of the holders of not less than a majority in aggregate principal amount of the outstanding debt securities affected by the modification or amendment. However, the following modifications and amendments will not be effective against any holder without its consent:

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a change in the stated maturity date of any payment of principal or interest;



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a reduction in the principal amount of, or interest on, any debt securities;



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an alteration or impairment of any right to convert at the rate or upon the terms provided in the indenture;



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a change in the currency in which any payment on the debt securities is payable;



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an impairment of a holder's right to sue us for the enforcement of payments due on the debt securities; or



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a reduction in the percentage of outstanding debt securities required to consent to a modification or amendment of the indenture or required to consent to a waiver of compliance with certain provisions of the indenture or certain defaults under the indenture.

Unless we indicate otherwise in the applicable prospectus supplement, if an event of default occurs and continues, the trustee or the holders of not less than 25% in aggregate principal amount of the outstanding debt securities of such series may declare the entire principal of all the debt securities to

However, the holder of any debt security will have an absolute right to receive payment of principal of, and interest on, the debt security when due and to institute suit to enforce payment.

Satisfaction and Discharge; Defeasance

Satisfaction and Discharge of Indenture. Unless otherwise indicated in the applicable prospectus supplement, if at any time,

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we have paid the principal of and interest on all the debt securities of any series, except for debt securities which have been destroyed, lost or stolen and which have been replaced or paid in accordance with the indenture, as and when the same has become due and payable;



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we have delivered to the trustee for cancellation all debt securities of any series theretofore authenticated, except for debt securities of such series which have been destroyed, lost or stolen and which have been replaced or paid as provided in the indenture; or

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all the debt securities of such series not theretofore delivered to the trustee for cancellation have become due and payable, or are by their terms are to become due and payable within one year or are to be called for redemption within one year, and we have irrevocably deposited with the trustee, in trust, sufficient money or government obligations, or a combination thereof, to pay the principal, any interest and any other sums due on the debt securities, on the dates the payments are due or become due under the indenture and the terms of the debt securities;

then the indenture shall cease to be of further effect with respect to the debt securities of such series, except for (a) rights of registration of transfer and exchange, and our right of optional redemption, (b) substitution of mutilated, defaced, destroyed, lost or stolen debt securities, (c) rights of holders to receive payments of principal thereof and interest thereon upon the original stated due dates therefor (but not upon acceleration) and remaining rights of the holders to receive mandatory sinking fund payments, if any, (d) the rights, powers, trusts, duties and immunities of the trustee under the indenture and our obligations in connection therewith, and (e) the rights of the holders of such series of debt securities as beneficiaries thereof with respect to the property so deposited with the trustee payable to all or any of them.

Defeasance and Covenant Defeasance.Unless otherwise indicated in the applicable prospectus supplement, we may elect with respect to any debt securities of any series either:

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to defease and be discharged from all of our obligations with respect to such debt securities ("defeasance"), with certain exceptions described below; or



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to be released from our obligations with respect to such debt securities under such covenants as may be specified in the applicable prospectus supplement, and any omission to comply with those obligations will not constitute a default or an event of default with respect to such debt securities ("covenant defeasance").

We must comply with the following conditions before the defeasance or covenant defeasance can be effected:

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we must irrevocably deposit with the indenture trustee or other qualifying trustee, trust funds in trust solely for the benefit of the holders of such debt securities, sufficient money or government obligations, or a combination thereof, to pay the principal, any interest and any other sums on the due dates for those payments;



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we must deliver to the trustee an opinion of counsel to the effect that the holders of such debt securities will not recognize income, gain or loss for federal income tax purposes as a result of such defeasance or covenant defeasance, as the case may be, to be effected with respect to such debt securities and will be subject to federal income tax on the same amount, in the same manner and at the same times as would be the case if such defeasance or covenant defeasance, as the case may be, had not occurred; and



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we must deliver to the trustee an officers' certificate and opinion of counsel stating that all conditions precedent relating to such defeasance or covenant defeasance, as the case may be, have been complied with.

In connection with defeasance, any irrevocable trust agreement contemplated by the indenture must include, among other things, provision for (a) payment of the principal of and interest on such debt securities, if any, appertaining thereto when due (by redemption, sinking fund payments or otherwise), (b) the payment of the expenses of the trustee incurred or to be incurred in connection with carrying out such trust provisions, (c) rights of registration, transfer, substitution and exchange of such debt securities in accordance with the terms stated in the indenture, and (d) the rights, powers, trusts, duties and immunities of the trustee under the indenture and our obligations in connection therewith as stated in the indenture.

Resignation or Removal of Trustee. If the trustee has or acquires a conflicting interest within the meaning of the Trust Indenture Act, the trustee must either eliminate its conflicting interest or resign, to the extent and in the manner provided by, and subject to the provisions of, the Trust Indenture Act and the indenture. Any resignation will require the appointment of a successor trustee under the indenture in accordance with the terms and conditions of the indenture.

The trustee may resign or be removed by us with respect to one or more series of debt securities and a successor trustee may be appointed to act with respect to any such series. The holders of a majority in aggregate principal amount of the debt securities of any series may remove the trustee with respect to the debt securities of such series.

Annual Trustee Report to Holders of Debt Securities. The trustee will be required to submit certain reports to the holders of the debt securities regarding, among other things, the trustee'strustee’s eligibility to serve as such, the priority of the trustee'strustee’s claims regarding advances made by it, and any action taken by the trustee materially affecting the debt securities.

Certificates and Opinions to Be Furnished to Trustee. The indenture provides that, in addition to other certificates or opinions specifically required by other provisions of the indenture, every application by us for action by the trustee must be accompanied by a certificate from one or more of our officers and an opinion of counsel (who may be our counsel) stating that, in the opinion of the signers, all conditions precedent to such action have been complied with by us.

TableDescription of ContentsWarrants

DESCRIPTION OF WARRANTS
In this section “Description of Warrants,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

We may issue warrants for the purchase of debt securities, preferred stock, common stock, other securities of the Company or any combination of the foregoing. Warrants may be issued alone or together with securities offered by any prospectus supplement and may be attached to, or separate from, those securities. We will issue each series of warrants under a separate warrant agreement and/or warrant certificate. The following outlines some of the general terms and provisions of the warrants. Further terms of the warrants and the applicable warrant agreement and/or warrant certificate will be stated in the applicable prospectus supplement. The following description and any description of the warrants in a prospectus supplement are not complete and are subject to and qualified in itstheir entirety by reference to the terms and provisions of the warrant agreement and/or warrant certificate, which we will file with the SEC in connection with an issuance of any warrants.

The applicable prospectus supplement will include some or all of the following information:

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the title and specific designation of the warrants;



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the aggregate number of warrants offered;



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the amount of warrants outstanding, if any;



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the designation, number and terms of the securities purchasable upon exercise of the warrants, and procedures that will result in the adjustment of those numbers;



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the exercise price or prices of the warrants;



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the dates or periods during which the warrants are exercisable;



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the designation and terms of any securities with which the warrants are issued;



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provisions for changes to or adjustments in the exercise price of the warrants;



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if the warrants are issued as a unit with another security, the date, if any, on and after which the warrants and the other security will be separately transferable;



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if the exercise price is not payable in U.S. dollars, the foreign currency, currency unit or composite currency in which the exercise price is denominated;



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any minimum or maximum amount of warrants that may be exercised at any one time;



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the anti-dilution, redemption or call provisions of the warrants, if any;



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if applicable, the identity of the warrant agent for the warrants and of any other depositaries, execution or paying agents, transfer agents, registrars or other agents;



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any terms, procedures and limitations relating to the transferability, exchange or exercise of the warrants; and



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any other material terms of the warrants.

Before exercising their warrants, holders of warrants will not have any of the rights of holders of the securities purchasable upon such exercise, including the right to receive dividends, if any, or payments upon our liquidation, dissolution or winding-up, or to exercise voting rights, if any.

DESCRIPTION OF DEPOSITARY SHARES
Description of Depositary Shares

In this section “Description of Depositary Shares,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

The following is a summary of the general terms of the deposit agreement to govern any depositary shares we may offer representing fractional interests in shares of our preferred stock, the depositary shares themselves and the related depositary receipts. A copy of the deposit agreement and form of depositary receipt relating to any depositary shares we issue will be filed with the SEC as an

The depositary shares will be evidenced by depositary receipts issued under the deposit agreement. If you purchase fractional interests in shares of the related series of preferred stock, you will receive depositary receipts as described in the applicable prospectus supplement.

Unless we specify otherwise in the applicable prospectus supplement, you will not be entitled to receive the whole shares of preferred stock underlying the depositary shares.

Dividend Rights

The depositary will distribute all cash dividends or other cash distributions in respect of the preferred stock underlying the depositary shares to each record holder of depositary shares based on the number of depositary shares owned by that holder on the relevant record date. The depositary will distribute only that amount which can be distributed without attributing to any holder of depositary shares a fraction of one cent, and any balance not so distributed will be added to and treated as part of the next sum received by the depositary for distribution to record holders of depositary shares.

If there is a distribution other than in cash, the depositary will distribute property to the entitled record holders of depositary shares, unless the depositary determines that it is not feasible to make that distribution. In that case the depositary may, with our approval, adopt the method it deems equitable and practicable for making that distribution, including any sale of property and distribution of the net proceeds from this sale to the applicable holders.

The deposit agreement will also contain provisions relating to how any subscription or similar rights offered by us to holders of the preferred stock will be made available to the holders of depositary shares.

Taxation

Owners of depositary shares will be treated for U.S. federal income tax purposes as if they were owners of the preferred stock represented by the depositary shares. If necessary, the applicable prospectus supplement will provide a description of U.S. federal income tax consequences relating to the purchase and ownership of the depositary shares and the preferred stock represented by the depositary shares.

Amendment and Termination of the Deposit Agreement

The form of depositary receipt evidencing the depositary shares and any provision of the deposit agreement may be amended by agreement between us and the depositary at any time. However, certain amendments as specified in the applicable prospectus supplement will not be effective unless approved by the record holders of at least a majority of the depositary shares then-outstanding. A deposit agreement may be terminated by us or the depositary only if:

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all outstanding depositary shares relating to the deposit agreement have been redeemed; or



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there has been a final distribution on the preferred stock of the relevant series in connection with our liquidation, dissolution or winding up of our business and the distribution has been distributed to the holders of the related depositary shares.

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Charges of Depositary

We will pay all transfer and other taxes and governmental charges arising solely from the existence of the depositary arrangements. We will pay associated charges of the depositary for the initial deposit of the preferred stock and any redemption of the preferred stock. Holders of depositary shares will pay transfer and other taxes and governmental charges and any other charges that are stated to be their responsibility in the deposit agreement.

Resignation and Removal of Depositary

The depositary may resign at any time by delivering notice to us. We may also remove the depositary at any time. Resignations or removals will take effect when a successor depositary is appointed, and it accepts the appointment.

DESCRIPTION OF SUBSCRIPTION RIGHTS
Description of Subscription Rights

In this section “Description of Subscription Rights,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

The following is a summary of the general terms of the subscription rights to purchase common stock or other securities that we may offer to stockholders using this prospectus.

Subscription rights may be issued independently or together with any other security and may or may not be transferable. As part of any subscription rights offering, we may enter into a standby underwriting or other arrangement under which the underwriters or any other person would purchase any securities that are not purchased in such subscription rights offering. If we issue subscription rights, they will be governed by a separate subscription agent agreement that we will sign with a bank or trust company, as rights agent, that will be named in the applicable prospectus supplement. The rights agent will act solely as our agent and will not assume any obligation to any holders of subscription rights certificates or beneficial owners of subscription rights.

The prospectus supplement relating to any subscription rights we offer will describe the specific terms of the offering and the subscription rights, including the record date for stockholders entitled to the subscription rights distribution, the number of subscription rights issued and the number of shares of common stock or other securities that may be purchased upon exercise of the subscription rights, the exercise price of the subscription rights, the date on which the subscription rights will become effective and the date on which the subscription rights will expire, and

any material U.S. federal income tax considerations. The following description and any description in a prospectus supplement is a summary only and is subject to, and qualified in its entirety by reference to the terms and provisions of the subscription rights, which we will file with the SEC in connection with an issuance of subscription rights.

In general, a subscription right entitles the holder to purchase for cash a specific number of shares of common stock or other securities at a specified exercise price. The rights are normally issued to stockholders as of a specific record date, may be exercised only for a limited period of time and become void following the expiration of such period. If we determine to issue subscription rights, we will accompany this prospectus with a prospectus supplement that will describe, among other things:

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the record date for stockholders entitled to receive the subscription rights;



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the number of shares of common stock or other securities that may be purchased upon exercise of each subscription right;



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the exercise price of the subscription rights;



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whether the subscription rights are transferable;



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the period during which the subscription rights may be exercised and when they will expire;



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the steps required to exercise the subscription rights;

The applicable prospectus supplement will describe the terms of any Stock Purchase Contracts or Stock Purchase Units. Any description in a prospectus supplement is a summary only and is subject to, and qualified in its entirety by reference to the terms and provisions of the Stock Purchase Contracts, and, if applicable, collateral or depositary arrangements, relating to the Stock Purchase Contracts or Stock Purchase Units, which we will file with the SEC in connection with an issuance of the Stock Purchase Contracts or Stock Purchase Units. Material U.S. federal income tax considerations applicable to the Stock Purchase Units and the Stock Purchase Contracts will also be discussed in the applicable prospectus supplement.

DESCRIPTION OF UNITS
Description of Units

In this section “Description of Units,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

As specified in the applicable prospectus supplement, we may issue units consisting of one or more debt securities, shares of common stock, shares of preferred stock, depositary shares, warrants or other securities, or any combination of such securities, including guarantees of any securities.

A prospectus supplement and any other offering materials relating to any units issued under the registration statement containing this prospectus will specify the terms of the units, including:

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the terms of the units and of any of the debt securities, common stock, preferred stock, depositary shares, warrants and guarantees comprising the units, including whether and under what circumstances the securities comprising the units may be traded separately;

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a description of the terms of any unit agreement governing the units; and

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a description of the provisions for the payment, settlement, transfer or exchange of the units.

Plan of the units and of any of the debt securities, common stock, preferred stock, depositary shares, warrants and guarantees comprising the units, including whether and under what circumstances the securities comprising the units may be traded separately;



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a description of the terms of any unit agreement governing the units; and



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a description of the provisions for the payment, settlement, transfer or exchange of the units.

Only underwriters that we have named in a prospectus supplement will be underwriters of the securities offered by that prospectus supplement.

If underwriters are used in the sale, they will acquire the securities for their own account and may resell the securities from time to time in one or more transactions at a fixed public offering price or at varying prices determined at the time of sale. The obligations of the underwriters to purchase the securities will be subject to the conditions set forth in the applicable underwriting agreement. We may offer the securities to the public through underwriting syndicates represented by managing underwriters or by underwriters without a syndicate. Subject to certain conditions, the underwriters will be obligated to purchase all of the securities offered by the prospectus supplement, other than securities covered by any over-allotment option. Any public offering price and any discounts or concessions allowed or reallowed or paid to dealers may change from time to time. We may use underwriters with whom we have a material relationship. We will describe in the prospectus supplement, naming the underwriter, the nature of any such relationship.

We may sell securities directly or through agents we designate from time to time. We will name any agent involved in the offering and sale of securities and we will describe any commissions we will pay the agent in the prospectus supplement. Unless the prospectus supplement states otherwise, our agent will act on a best-efforts basis for the period of its appointment.

We may authorize agents or underwriters to solicit offers by certain types of institutional investors to purchase securities from us at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. We will describe the conditions to these contracts and the commissions we must pay for solicitation of these contracts in the applicable prospectus supplement.

We may provide agents and underwriters with indemnification against civil liabilities, including liabilities under the Securities Act, or contribution with respect to payments that the agents or

LEGAL MATTERS
Legal Matters

Certain legal matters in connection with any offering of securities made by this prospectus will be passed upon for us by our counsel Nelson Mullins Riley & Scarborough LLP of Atlanta, Georgia. If the securities are being distributed in an underwritten offering, certain legal matters will be passed upon for the underwriters by counsel identified in the related prospectus supplement.

EXPERTS
Experts

The financial statements incorporated in this prospectus by reference to the Annual Report on Form 10-K for the year ended December 31, 2016,2022, and the effectiveness of our internal controls over financial reporting as of December 31, 2016,2022, have been so incorporated in reliance on the report of Plante & Moran PLLC, independent registered public accounting firm, given on their authority as experts in auditing and accounting.

WHERE YOU CAN FIND MORE INFORMATION
Where You Can Find More Information

We are subject to the informational requirements of the Exchange Act and file with the SEC proxy statements, Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q and Current Reports on Form 8-K, as required of a U.S. listed company. You may read and copy any document we file at the SEC's public reference room at 100 F Street, N.E., Room 1580, Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information on the public reference room.8-K. Our SEC filings are also available to the public from the SEC'sSEC’s web site atwww.sec.gov or on our website atwww.oldsecond.com. However, other than our available SEC filings, the information on, or that can be accessible through, our website does not constitute a part of, and is not incorporated by reference in, this prospectus. Written requests for copies of the documents we file with the SEC should be directed to Old Second Bancorp, Inc., 37 South River Street, Aurora, Illinois 60507, Attention: Corporate Secretary, telephone: (630) 892-0202.

We will provide to each person, including any beneficial owner, to whom a prospectus is delivered, without charge, upon written or oral request, a copy of any or all of the documents that are incorporated by reference into this prospectus and a copy of any or all other contracts or documents which are referred to in this prospectus. Requests should be directed to:

Old Second Bancorp, Inc.

Attention: Corporate Secretary

37 South River Street

Aurora, Illinois 60507

Telephone number: (630) 892-0202

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PART II
INFORMATION NOT REQUIRED IN PROSPECTUS

Item 14. Other Expenses of Issuance and Distribution

The following table sets forth our estimated expenses to be incurred in connection with the issuance and distribution of the securities being registered, other than underwriting discounts and commissions. We will pay all of these expenses.

Item 15. Indemnification of Directors and Officers

Delaware Law. Section 145 of the Delaware General Corporation Law permits a corporation to indemnify any person who was or is a party or is threatened to be made a party to any threatened, pending or completed action, suit or proceeding, whether civil, criminal, administrative or investigative, by reason of the fact that the person is or was a director, officer, employee or agent of the corporation or another enterprise if serving at the request of the corporation. Depending on the character of the proceeding, a corporation may indemnify against expenses (including attorneys'attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred in connection with such action, suit or proceeding if the person indemnified acted in good faith and in a manner reasonably believed to be in or not opposed to the best interests of the corporation and, with respect to any criminal action or proceeding, had no reasonable cause to believe the person'sperson’s conduct was unlawful. In the case of an action by or in the right of the corporation, no indemnification may be made with respect to any claim, issue or matter as to which such person shall have been adjudged to be liable to the corporation unless and only to the extent that the Delaware Court of Chancery or the court in which such action or suit was brought shall determine that, despite the adjudication of liability, such person is fairly and reasonably entitled to indemnity for such expenses which the court shall deem proper. Section 145 further provides that to the extent a director or officer of a corporation has been successful on the merits or in the defense of any action, suit or proceeding referred to above, or in the defense of any claim, issue or matter therein, he or she shall be indemnified against expenses (including attorneys'attorneys’ fees) actually and reasonably incurred by him or her in connection therewith.

Certificate of Incorporation and Bylaws. Article VIII of our Certificate of Incorporation provides that we shall, to the full extent permitted by Section 145 of the Delaware General Corporation Law, indemnify all persons whom we may indemnify pursuant thereto. Section 8.8 of our Bylaws contains indemnification provisions substantially similar to Section 145 of the Delaware General Corporation Law.

Liability Insurance. We have obtained directors'directors’ and officers'officers’ liability insurance. The policy provides for coverage including prior acts and liabilities under the Securities Act, within the limits and subject to the limitations of such insurance.

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