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                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                    FORM 8-K

                CURRENT REPORT PURSUANT TO SECTION 13 OR 15(d) OF
                       THE SECURITIES EXCHANGE ACT OF 1934

                                 January 6, 1999
                Date of Report (Date of earliest event reported)

                            OSI PHARMACEUTICALS, INC.
             (Exact name of registrant as specified in its charter)

            DELAWARE                  0-15190                    13-3159796
(State or other jurisdiction of     (Commission               (I.R.S. Employer
         incorporation)             File Number)             Identification No.)

                         106 CHARLES LINDBERGH BOULEVARD
                               UNIONDALE, NY 11553
                    (Address of principal executive offices)

                                 (516) 222-0023
              (Registrant's telephone number, including area code)

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ITEM 5. OTHER EVENTS

         On January 6, 1999, the Board of Directors of OSI Pharmaceuticals,
Inc., a Delaware corporation (the "Company") adopted a Shareholders Rights Plan
and certain amendments to the By-Laws of the Company.

         The By-Law amendments (1) provide exclusive authority to the Board of
Directors to call a special meeting of stockholders of the Company (see Article
I, Section 1.2 of the Amended and Restated By-Laws), and (2) create advance
notice requirements and procedures for the submission by stockholders of
nominations for the Board of Directors and stockholder proposals (see Article I,
Section 1.8 of the Amended and Restated By-Laws). The full text of the Amended
and Restated By-Laws is attached hereto as Exhibit 99.1 and incorporated herein
by reference.

         In connection with the adoption of the Shareholders Rights Plan, the
Company declared a dividend distribution of one right (a "Right") for each
outstanding share of common stock, par value $.01 per share (the "Common Stock")
of the Company payable to stockholders of record at the close of business on
February 15, 1999 (the "Record Date"). One Right will also be issued to the
holder of each share of Common Stock that becomes outstanding after the Record
Date and before the earliest of (i) the Distribution Date, as defined below,
(ii) the redemption of the Rights, and (iii) the expiration of the Rights.
Except as set forth below, and subject to adjustment as provided in the Rights
Agreement (as defined below), each Right entitles the registered holder to
purchase from the Company one Unit, consisting of one one-tenth share of Common
Stock and one Subordinated Debenture in the principal amount equal to
nine-tenths of the current per share market price of the Common Stock on the
date of exercise, for an exercise price of $50.00 per Right (the "Purchase
Price"). Under certain circumstances, as set forth in the form of Rights
Agreement, the Company may substitute other securities or property for the Unit
otherwise obtainable upon exercise of a Right, provided the value of such
securities is equal to the then current market price of one share of Common
Stock. The description and terms of the Rights are set forth in the Rights
Agreement (the "Rights Agreement") dated as of January 6, 1999 between the
Company and The Bank of New York, as Rights Agent (the "Rights Agent"). The
Subordinated Debentures will be issuable under, and subject to the terms and
conditions of, an Indenture (the "Indenture") between the Company and The Bank
of New York, as Trustee.

         The Rights are not exercisable until the "Distribution Date" which is
the earlier of (i) the date which is 10 days (or such later date as the Board of
Directors may determine) after the commencement of, or first public announcement
of an intention to make, a tender or exchange offer by any person or entity
(each a "Person"), other than the Company and certain related entities, if, upon
the consummation of such offer a Person or group of affiliated or associated
Persons would become an Acquiring Person (defined below) or (ii) the date of the
first public announcement that a Person or group of affiliated or associated
Persons has acquired, or obtained the right to acquire, otherwise than pursuant
to a Permitted Offer (defined below), beneficial ownership of 17.5% or more of
the outstanding shares of Common Stock. A Person or group whose acquisition of
shares of Common Stock causes a Distribution Date to occur pursuant to the
foregoing clause (ii)


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is an "Acquiring Person."

         Until the Distribution Date (or earlier redemption or expiration of the
Rights) (i) the Rights will be evidenced by the Common Stock certificates and
not by separate certificates and will be transferred with and only with such
Common Stock certificates (except in connection with the redemption of the
Rights), (ii) new Common Stock certificates issued after the Record Date will
contain a notation referring to the Rights associated with such shares of Common
Stock, incorporating the Rights Agreement by reference and (iii) the surrender
for transfer of any certificates for Common Stock outstanding will also
constitute the transfer of the Rights associated with the Common Stock
represented by such certificates. As soon as practicable after the Distribution
Date, separate Right Certificates will be mailed to holders of record of the
Common Stock as of the close of business on the Distribution Date and,
thereafter, the separate Right Certificates alone will represent the Rights.

         The Rights will expire at the close of business on January 5, 2009,
unless earlier redeemed by the Company as described below.

         In the event that any Person becomes an Acquiring Person (otherwise
than pursuant to a Permitted Offer), the Rights will be modified automatically
so that each holder of a Right will thereafter have, in lieu of the right to
purchase a Unit, the right (the "Flip-In Right") to receive upon exercise of the
Right the number of shares of Common Stock which, immediately before such
Acquiring Person became an Acquiring Person, had a market value equal to twice
the amount of the exercise price of the Right. Notwithstanding the foregoing,
after such Person shall have become an Acquiring Person, all Rights that are, or
under certain circumstances specified in the Rights Agreement were, beneficially
owned by any Acquiring Person or any affiliate or associate thereof will be null
and void, and any holder of such Rights will have no right to exercise such
Rights. A "Permitted Offer" is a tender or exchange offer which is for all
outstanding shares of Common Stock at a price and on terms which the Board of
Directors determines to be adequate and in the best interests of the Company,
its stockholders and other relevant constituencies, other than such Acquiring
Person, its affiliates and associates.

         In the event that, at any time after a Person or group has become an
Acquiring Person, (i) the Company is acquired in a merger or other business
combination in which the holders of all of the outstanding shares of Common
Stock immediately prior to the consummation of the transaction are not the
holders of all of the surviving corporation's voting power or (ii) more than 50%
of the Company's assets or earning power is sold, leased, exchanged, mortgaged,
pledged or otherwise transferred or disposed of (in one transaction or a series
of related transactions), in any such case with or to an Acquiring Person or any
affiliate or associate thereof or any other Person in which such Acquiring
Person, affiliate or associate has an interest or any Person acting on behalf of
or in concert with such Acquiring Person (or, if in such transaction all holders
of shares of Common Stock are not treated alike, any other Person), then each
holder of a Right (except Rights which previously have become null and void as
set forth above) shall thereafter have the right (the "Flip-Over Right") to
receive, upon exercise of the Right, shares of common stock of the acquiring
company having a value equal to twice the amount of the exercise price of the
Right. Each such holder of a Right will continue to have a Flip-Over Right
whether


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or not such holder exercises or surrenders the Flip-In Right, and such holder
will have a successive Flip-Over Right on each occurrence of a transaction
specified in the first sentence of this paragraph.

         At any time before a Person or group becomes an Acquiring Person, the
Company may redeem the Rights in whole, but not in part, at a price of $.001 per
Right (the "Redemption Price"), subject to adjustment. Immediately upon the
action of the Board of Directors ordering redemption of the Rights, the right to
exercise the Rights will terminate and the only right of the holders of Rights
will be to receive the Redemption Price. Notice of redemption will be given by
mail to each holder of Rights at such holder's last address on the registry
books for the Rights, or, at the Company's option, by issuing a press release
and mailing payment of the redemption price to the registered holders of the
Rights.

         The Purchase Price payable, and the number and kind of securities or
property issuable, upon exercise of the Rights are subject to adjustment from
time to time to prevent dilution in the event of certain changes in the Common
Stock or distributions of such stock or other events which would otherwise
diminish the benefits intended to be afforded by the Rights. With certain
exceptions, no adjustment in the Purchase Price will be required until
cumulative adjustments amount to at least 1% of the Purchase Price.

         No fractional Rights, shares of Common Stock or Subordinated Debentures
will be issued (other than fractions of a share of Common Stock which are one
one-tenth of a share or an integral multiple of one one-tenth of a share) and,
in lieu thereof, a payment in cash will be made based on the market price of the
Rights, Common Stock or Subordinated Debentures, as the case may be, on the last
trading date prior to the date of exercise of the Rights involved or the Company
may issue scrip, warrants or depositary receipts. Subordinated Debentures will
be issued in denominations not less than $10 or integral multiples thereof and
the Company may pay cash in lieu of issuing Subordinated Debentures in
denominations other than $10 or integral multiples thereof in an amount equal to
the difference between the principal amount of the Subordinated Debentures
otherwise issuable and the integral multiple of $10 which is nearest to, but not
in excess of, such amount.

         The terms of the Subordinated Debentures are designed, and the interest
rate will be set, so that the value of the one Unit issuable upon exercise of
each Right will approximate the value of one share of Common Stock and the
Subordinated Debentures will trade when issued at par. The payment of principal
and interest on account of the Subordinated Debentures will be subordinate to
the repayment in full of all senior indebtedness (including interest thereon) of
the Company.

         Prior to the Distribution Date, the Board of Directors may, without the
approval of any holder of Rights, supplement or amend any provision of the
Rights Agreement other than to decrease the Redemption Price, increase the
Exercise Price, or decrease the number of Units, fractional shares of Common
Stock or the calculation of the principal amount or interest rate of the
Subordinated Debentures constituting such Units, or decrease the amount of other
securities, cash or property for which a Right is exercisable. After the
Distribution Date, the provisions of


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the Rights Agreement may be amended by the Board of Directors only to cure any
ambiguity, defect or inconsistency, to make changes which do not adversely
affect the interests of the holders of Rights (other than holders whose Rights
have become null and void as set forth above) or to shorten or lengthen any time
period under the Rights Agreement; provided that no such amendment may be
adopted to adjust the time period governing redemption at a time when the Rights
are not redeemable.

         Until a Right is exercised, the holder thereof, as such, will have no
rights as a stockholder or Subordinated Debentureholder of the Company,
including, without limitation, the right to vote or to receive dividends, other
distributions, or payments of principal or interest. While the distribution of
the Rights will not be taxable to stockholders of the Company, stockholders may,
depending upon the circumstances, recognize taxable income should the Rights
become exercisable or upon the occurrence of certain events thereafter.

         The summary description of the Rights set forth herein does not purport
to be complete and is qualified in its entirety by reference to the Rights
Agreement, which is filed as Exhibit 4 hereto.


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ITEM 7.  EXHIBITS



- ----------------------- ---------------------------------------------------------------------------------------------------
     EXHIBIT NO.                                                   DESCRIPTION
- ----------------------- ---------------------------------------------------------------------------------------------------
                     
          4             Form of Rights Agreement, dated as of January 11, 1999, between OSI Pharmaceuticals, Inc. and The
                        Bank of New York, as Rights Agent, including Summary of Rights to Purchase Common Stock and
                        Subordinated Debenture (Exhibit A thereto) and Form of Right Certificate (Exhibit B thereto).
- ----------------------- ---------------------------------------------------------------------------------------------------
         99.1           Amended and Restated By-Laws of OSI Pharmaceuticals, Inc.
         99.2           Press release, dated January 8, 1999
- ----------------------- ---------------------------------------------------------------------------------------------------


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                                    SIGNATURE

         Pursuant to the requirements of the Securities Exchange Act of 1934,
the registrant has duly caused this report to be signed on its behalf by the
undersigned hereunto duly authorized.

Date:  January 8, 1999          OSI PHARMACEUTICALS, INC.

                             By: /s/ Robert L. Van Nostrand
                                 Name: Robert L. Van Nostrand
                                 Title: Vice President and Chief Financial
                                           Officer, Secretary and Treasurer

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                                  EXHIBIT INDEX



- ----------------------- ---------------------------------------------------------------------------------------------------
     EXHIBIT NO.                                                   DESCRIPTION
- ----------------------- ---------------------------------------------------------------------------------------------------
                     
          4             Form of Rights Agreement, dated as of January 6, 1999, between OSI Pharmaceuticals, Inc. and The
                        Bank of New York including Summary of Rights to Purchase Common Stock and Subordinated Debenture
                        (Exhibit A thereto) and Form of Right Certificate (Exhibit B thereto).
- ----------------------- ---------------------------------------------------------------------------------------------------
         99.1           Amended and Restated By-Laws of OSI Pharmaceuticals, Inc.
         99.2           Press release, dated January 8, 1999.
- ----------------------- ---------------------------------------------------------------------------------------------------