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 Tuesday,  January 3, 2006,  the closing
price of the Common Stock as reported on the New York Stock  Exchange was $86.11
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 Beneficially        Amount of   
                                             Owned as of          Shares to be  
Name of Selling Shareholder                January 3, 2006      Offered for Sale
---------------------------              -------------------    ----------------
    Gail Margaret Ruddy                           325                 325

      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 5, 2006.