EXHIBIT 4(o)

                                                  TORONTO DOMINION (TEXAS), Inc.

                                                  Suite 1700\909 Fannin
                                                  Houston, Texas 77010

                                                  Telephone No.

                                December 12, 2000


Safety-Kleen Services, Inc.
1301 Gervais Street, Suite 300
Columbia, South Carolina 29201
Attention:  Larry Singleton

Safety-Kleen Systems, Inc.
1301 Gervais Street, Suite 300
Columbia, South Carolina 29201
Attention:  Larry Singleton

          Re: Amended and Restated Marketing and Distribution Agreement
              ---------------------------------------------------------

Ladies and Gentlemen:

         Reference  is hereby  made to (i) Amended and  Restated  Marketing  and
Distribution   Agreement  dated  as  of  December  14,  2000  (the   "System-One
Agreement")  by  and  among   Safety-Kleen   Systems,   Inc.,  a  subsidiary  of
Safety-Kleen Corp., with offices located at 1301 Gervais Street, Columbia, South
Carolina 29201 ("SK Systems") and System One Technologies Inc.,  ("System-One"),
a Florida corporation, with offices located at 8305 N.W. 27th Street, Suite 107,
Miami,  Florida  33122 and (ii) the Amended and  Restated  Debtor in  Possession
Credit Agreement,  initially dated as of June 11, 2000,  amended and restated as
of July 19, 2000 (the "DIP Credit Agreement") among Safety-Kleen Services, Inc.,
a Delaware Corporation (together with SK Systems, "Safety-Kleen"), the financial
institutions or entities from time to time parties thereto (the "Lenders"),  The
Toronto Dominion Bank, Houston Agency, as letter of credit issuing bank, Toronto
Dominion  (Texas) Inc.,  as  administrative  agent (the "General  Administrative
Agent") and the CIT Business  Group/Business Credit Inc. as collateral agent and
underwriter  (the "Collateral  Agent").  Unless  otherwise  defined herein,  all
capitalized  terms used herein  shall have the  meaning  ascribed to them in the
System-One Agreement.


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         Absent the express prior written consent of the General  Administrative
Agent,  Safety-Kleen  hereby  acknowledges  and agrees  that the total US dollar
amount of  $20,000,000.  Safety-Kleen  further  acknowledges  and agrees that a
violation of this letter  agreement  will  constitute  an event of default under
section 9 of the DIP credit.

                                         Very Truly Yours,

                                         Toronto Dominion (Texas), Inc.



                                         By:  /s/ Warren Finlay
                                              ------------------------------
                                              Name:   Warren Finlay
                                              Title   President

Agreed to and accepted as of the
date first above written:

SAFETY-KLEEN SERVICES, INC.


By:  /s/ Larry W. Singleton
     -----------------------------------
     Name:   Larry W. Singleton
     Title:  C.F.O.

SAFETY-KLEEN SYSTEMS, INC.


By:  /s/ Larry W. Singleton
     -----------------------------------
     Name:   Larry W. Singleton
     Title:  C.F.O.