PROSPECTUS SUPPLEMENT
(To Prospectus Dated March 6, 1998)

                               OMNICOM GROUP INC.
                        $230,000,000 Principal Amount of
               2 1/4% Convertible Subordinated Debentures due 2013
                     (Interest Payable January 6 and July 6)
                        4,615,694 Shares of Common Stock

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      This document  supplements the Prospectus  dated March 6, 1998 relating to
(i) $230,000,000  aggregate principal amount of 2 1/4% Convertible  Subordinated
Debentures  due 2013  (the  "Debentures")  of  Omnicom  Group  Inc.,  a New York
corporation  ("Omnicom" or the "Company"),  and (ii) 4,615,694  shares of Common
Stock,  par value $0.50 per share (the "Common  Stock") of the Company which are
initially   issuable  upon   conversion  of  the  Debentures   plus   additional
indeterminate  number  of shares of Common  Stock as may  become  issuable  upon
conversion of the Debentures as a result of adjustments to the conversion  price
(the  "Shares").  The  Debentures  were  initially  acquired from the Company by
Morgan Stanley & Co.  Incorporated  in January 1998 in connection with a private
offering.  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 November 18, 1998 the closing  price of the Common Stock as
reported on the New York Stock Exchange was $54.625 per share.  The Common Stock
is traded under the symbol "OMC".

      In  accordance  with  the  Section  of the  Prospectus  entitled  "Selling
Securityholders"  (which appears on pages 19 of the  Prospectus),  the following
information is provided with respect to the beneficial owners of the Debentures:

                                      Principal Amount of    Principal Amount of
                                    Debentures Beneficially    Debentures to be
Name of Selling Securityholder(s)            Owned             Offered for Sale
- ---------------------------------   -----------------------  -------------------
GLG Global Convertible Fund               $2,000,000              $2,000,000
Morgan Stanley Dean Witter                   500,000                 500,000

      The  Debentures  being  offered  by  the  Selling  Securityholders  hereby
represent   all  of  the   Debentures   beneficially   owned   by  the   Selling
Securityholders as of November 18, 1998. Except for Morgan Stanley's position as
the Initial Purchaser for the Debentures,  as initial purchaser of the Company's
4 1/4%  Convertible  Subordinated  Debentures due 2007 issued on January 3, 1997
and as lead underwriter of 4,000,000 shares of the Company's Common Stock issued
on March 9, 1998 and  except for the  purchase  of the  Debentures,  none of the
Selling  Securityholders has 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 November 20, 1998.