PROSPECTUS SUPPLEMENT
(To Prospectus Dated January 28, 1998)

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

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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  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,  originally issued in a private offering in Canada by
OCI for the Class C Special  Shares of Quintenco  Holdings  Inc., a  corporation
(now amalgamated with OCI)  incorporated  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 January 24, 2005, the
closing price of the Common Stock as reported on the New York Stock Exchange was
$84.06 per share. The Common Stock is traded under the symbol "OMC."

      The Selling  Shareholder  does not own in excess of 1% of the Common Stock
of the Company and since the Selling  Shareholder  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
Shareholder 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
Name of Selling                 Beneficially Owned                Shares to be
Shareholder                   as of January 24, 2005            Offered for Sale
---------------               ----------------------            ----------------

John Campbell Reston                  3,000                           3,000

      Except  for the  purchase  of the  shares of  Common  Stock,  the  Selling
Shareholder has 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 January 25, 2005.