PROSPECTUS SUPPLEMENT
(To Prospectus Dated January 28, 1998)

                        1,000,000 Shares of Common Stock
                                ($.15 Par Value)

                               OMNICOM GROUP INC.

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      This document  supplements the Prospectus  dated January 28, 1998 relating
to  1,000,000  shares of Common  Stock,  par value $.15 per share  (the  "Common
Stock") of the Omnicom  Group Inc.,  a New York  corporation  ("Omnicom"  or the
"Company").  Each share of Common Stock offered  hereby was issued upon exchange
of an  Exchangeable  Share (an  "Exchangeable  Share")  of Omnicom  Canada  Inc.
("OCI"), a corporation incorporated under the laws of Ontario and a wholly-owned
subsidiary of the Company, issued in a private offering in Canada by OCI for the
Class A Common  Shares and Class B Common Shares of Quintenco  Holdings  Inc., a
corporation  organized under the laws of Ontario ("QHI"), in connection with the
combination  of OCI and QHI.  This  Prospectus  Supplement  is  incorporated  by
reference into the Prospectus,  and all terms used herein shall have the meaning
assigned to them in the  Prospectus.  On June 21, 2000, the closing price of the
Common  Stock as reported on the New York Stock  Exchange was $95.375 per share.
The Common Stock is traded under the symbol "OMC."

      The Selling Shareholders do not own in excess of 1% of the Common Stock of
the Company and since the Selling Shareholders may sell all, some or none of the
shares of Common Stock offered hereby,  no estimate can be made of the aggregate
number of shares of Common Stock that will be owned by the Selling  Shareholders
upon completion of the offering to which this Prospectus  Supplement relates. In
accordance with the Section of the Prospectus  entitled  "Selling  Shareholders"
(which  appears on page 10 of the  Prospectus),  the  following  information  is
provided with respect to the beneficial owners of the Common Stock:

                                    Amount of Shares              Amount of
     Names of Selling              Beneficially Owned            Shares to be
     Shareholders                  as of June 21, 2000         Offered for Sale
     ----------------              -------------------         ----------------

     Patrick Joseph Walshe                1,768                      1,768

     1222287 Ontario Inc.                 5,119                      5,119

     John Campbell Reston                 3,937                      3,937

     1222286 Ontario Inc.                34,890                     34,890


      Except  for the  purchase  of the  shares of  Common  Stock,  the  Selling
Shareholders have not had a material relationship with the Company or any of its
affiliates within the past three years.

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      THESE  SECURITIES  HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES
AND EXCHANGE  COMMISSION OR ANY STATE SECURITY COMMISSION NOR HAS THE SECURITIES
AND  EXCHANGE  COMMISSION  OR ANY STATE  SECURITIES  COMMISSION  PASSED UPON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.

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            The date of this Prospectus Supplement is June 27, 2000.