PROSPECTUS SUPPLEMENT
(To Prospectus Dated March 11, 1997)

                               OMNICOM GROUP INC.
                        $218,500,000 Principal Amount of
               4 1/4% Convertible Subordinated Debentures due 2007
                     (Interest Payable January 3 and July 3)

                        3,468,254 Shares of Common Stock

                              ---------------------

     This document  supplements the Prospectus  dated March 11, 1997 relating to
(i) $218,500,000  aggregate principal amount of 4 1/4% Convertible  Subordinated
Debentures  due 2007  (the  "Debentures")  of  Omnicom  Group  Inc.,  a New York
corporation  ("Omnicom" or the "Company"),  and (ii) 3,468,254  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 1997 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 June 6,  1997  the  closing  price of the  Common  Stock as
reported on the New York Stock  Exchange was $61 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 and 20 of the  Prospectus),  the
following  information is provided with respect to the beneficial  owners of the
Debentures:

                                       Principal Amount of   
                                           Debentures        Principal Amount of
                                          Beneficially         Debentures to be 
Name of Selling Securityholder(s):            Owned            Offered for Sale 
- ----------------------------------            -----            ---------------- 
American Community Mutual Ins. Co.        $  180,000            $  180,000
American Pioneer Life Ins. Co. of N.Y.        45,000                45,000
American Progressive Life & Health            45,000                45,000
American Public Entity Excess Pool            50,000                50,000
American Republic Insurance Company          400,000               400,000
Amwest Surety Insurance Company              350,000               350,000
Anthracite Mutual Fire Ins. Company           10,000                10,000
Associated Physicians Ins. Co.                25,000                25,000
BCS Life Insurance Co.                       295,000               295,000
Care America Life Insurance Co.               50,000                50,000
Catholic Relief Ins. Company of America      210,000               210,000
Central States Health & Life of Omaha        130,000               130,000
Century National Insurance Company           725,000               725,000
Chicago Mutual Liability Co.                  40,000                40,000
Chrysler Insurance Co.                     1,440,000             1,440,000
Concord Life Ins. Co.                         30,000                30,000
Condor Insurance                             110,000               110,000
CSA Fraternal Life                            20,000                20,000
Delta Air Lines Master Trust               1,225,000             1,225,000
Farmers Home Mutual Insurance                100,000               100,000
Federated Rural Electric Ins. Corp.          100,000               100,000
First Mercury Ins. Company                   250,000               250,000
Frontier Insurance Company                   600,000               600,000
Goodville Mutual Casualty Company             30,000                30,000
Gopher State Mutual Ins. Company              80,000                80,000
Goschenhoppen-Home Ins. Co.                   40,000                40,000
Grain Dealers Mutual Ins.                    120,000               120,000
Guarantee Trust Life Ins. Co.                620,000               620,000
Guaranty Income Life Insurance               190,000               190,000
Highbridge Capital Corporation             5,250,000             5,250,000
Holy Family Society                           30,000                30,000
Hughes Aircraft Company Master 
Retirement Trust                             610,000               610,000
ISBA Mutual Insurance Company                100,000               100,000
Lebanon Mutual Ins. Company                   50,000                50,000
Lone Star Life Insurance Co.                 820,000               820,000
Medico Life Insurance Company                440,000               440,000
Medmarc Insurance                            400,000               400,000
Merrill Lynch Pierce Fenner and Smith Inc. 2,030,000             2,030,000
Middle Cities Risk Management Trust          130,000               130,000
Midwest Securities Life                      120,000               120,000
Midwestern National Life Ins. Co. of Ohio    250,000               250,000
Millers Casualty Ins. Co. of Texas           180,000               180,000
Millers Mutual Fire Ins. Co. of Texas        850,000               850,000
Millville Mutual Ins. Co.                     60,000                60,000
Mutual Protective Ins. Co.                   410,000               410,000
National Chiropractic                         80,000                80,000
OCM Convertible Trust                      1,730,000             1,730,000
Old Guard Fire Ins. Co.                      100,000               100,000
Old Guard Insurance Company                  270,000               270,000
Ozark National Life Ins. Co.                 440,000               440,000
Partner Reinsurance Company Ltd.             235,000               235,000
Phico Insurance Company                       60,000                60,000
Physicians Mutual Ins. Co.                   200,000               200,000
Pioneer Insurance Company                     15,000                15,000
Police & Fireman's Insurance Association      45,000                45,000
Reassurance Company of Hanover               280,000               280,000
Reliable Life Insurance Co.                  420,000               420,000
Salomon Brothers Inc.                      4,810,000             4,810,000
Secura Insurance, A Mutual Company           140,000               140,000
Service Life and Casualty Ins. Co.            25,000                25,000
Service Lloyd Insurance Company               25,000                25,000
Standard Mutual Ins. Co.                     160,000               160,000
State Employees' Retirement Fund of the
State of Delaware                            485,000               485,000
State of Connecticut Combined
Investment Funds                             285,000               285,000
State of Connecticut Combined
Investment Funds                           1,415,000             1,415,000
Texas Builders Insurance Co.                  50,000                50,000
Transguard Insurance Company of America      530,000               530,000
United National Insurance Co.              1,310,000             1,310,000
United Teacher Associates Ins. Co.           990,000               990,000
Utica First Insurance Company                 95,000                95,000
Vanguard Convertible Securities Fund, Inc. 1,015,000             1,015,000
Washington International Ins. Co.            200,000               200,000
Western Home Insurance Co.                   100,000               100,000
Westward Life Insurance Co.                   50,000                50,000
Wisconsin Lawyers Mutual Ins. Co.             50,000                50,000
Wisconsin Mutual Insurance Co.                30,000                30,000
World Insurance Company                      150,000               150,000

     The  Debentures  being  offered  by  the  Selling   Securityholders  hereby
represent   all  of  the   Debentures   beneficially   owned   by  the   Selling
Securityholders  as of June 6, 1997.  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.

                             -----------------------

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.

                             ----------------------

            The date of this Prospectus Supplement is June 10, 1997.