EXHIBIT 4

                            PARENT VOTING AGREEMENT


          PARENT VOTING AGREEMENT (this "Agreement"), dated as of January 15,
1998, among International Technology Corporation, a Delaware corporation
("Parent"), the undersigned stockholders (the "Stockholders") of Parent and OHM
Corporation, an Ohio corporation (the "Company").

          WHEREAS, concurrently with the execution of this Agreement, the
Company, Parent and IT-Ohio, Inc., an Ohio Corporation and a wholly owned
subsidiary of Parent ("Merger Sub"), have entered into an Agreement and Plan of
Merger (as it may be hereafter amended from time to time, the "Merger
Agreement"), which provides that Merger Sub shall make a tender offer (the
"Offer") for 13,933,000 shares of common stock of the Company and that Merger
Sub shall thereafter merge (the "Merger") with and into the Company pursuant to
the terms and conditions of the Merger Agreement, and sets forth certain
representations, warranties, covenants and agreements of the parties thereto in
connection with the Offer, the Merger and the other transactions contemplated
therein (the "Merger Transactions"); and

          WHEREAS, upon consummation the Merger, the holders of outstanding
shares of common stock of the Company at the Effective Time (as defined in the
Merger Agreement) will receive the Merger Consideration (as defined in the
Merger Agreement), which will consist (at least in part) of shares of Common
Stock, par value $0.1 per share, of Parent ("Parent Common Stock") for each
share of common stock of the Company; and

          WHEREAS, Section 6.4 of the Merger Agreement provides that if
stockholder approval of the issuance of shares of Parent Common Stock is
required under the rules of the New York Stock Exchange, Inc., Parent will take
all action necessary to convene a meeting of its stockholders as promptly as
practicable to consider and vote upon such issuance; and

          WHEREAS, the Stockholders are the record holders of an aggregate of
45,000 shares of Cumulative Convertible Participating Preferred Stock, par value
$100 per share, of Parent (the "Preferred Shares") and warrants (the "Warrants")
to purchase 1,250,000 shares of Parent Common Stock; and

          WHEREAS, in order to induce the Company, Parent and Merger Sub to
enter into the Merger Agreement, the Stockholders wish to agree (i) to vote the
Preferred Shares 

 
and any other shares of capital stock of Parent held by them so as to facilitate
the consummation of the Merger Transactions, (ii) except as provided in this
Agreement, not to transfer or otherwise dispose of any of the Preferred Shares
or, any other shares of capital stock held by them, or any other shares of
capital stock of Parent acquired by them hereafter and prior to the Effective
Time (as defined in the Merger Agreement), and (iii) to deliver to the Company
an irrevocable proxy to vote the Preferred Shares and any other shares of
capital stock held by them, and any other shares of capital stock of Parent
acquired by them hereafter and prior to the Effective Time, all in manner set
forth herein.

          NOW, THEREFORE, for good and valuable considera  tion, the receipt,
sufficiency and adequacy of which is hereby acknowledged, the parties hereto
agree as follows:

          1.   Representations of Stockholders.  Each of the Stockholders
               -------------------------------                           
represents and warrants to the Company, Parent and Merger Sub that (a) such
Stockholder lawfully owns beneficially (as such term is defined in the
Securities Exchange Act of 1934, as amended (the "1934 Act")), and of record the
number of Preferred Shares set forth opposite such Stockholder's name on Exhibit
A (such Stockholder's "Shares") free and clear of all liens, claims, charges,
security interests or other encumbrances and, except for this Agreement and the
Merger Agreement, there are no options, warrants or other rights, agreements,
arrangements or commitments of any character to which such Stockholder is a
party relating to the pledge, disposition or voting of any shares of capital
stock of Parent and there are no voting trusts or voting agreements with respect
to such Shares, (b) such Stockholder does not beneficially own any shares of
Parent Common Stock or any other shares of capital stock of Parent other than
such Shares and, except for such Shares, does not have any options, warrants or
other rights to acquire any additional shares of capital stock of Parent or any
security exercisable for or convertible into shares of capital stock of Parent,
other than the Warrants, (c) such Stockholder has full power and authority to
enter into, execute and deliver this Agreement and to perform fully such
Stockholder's obligations hereunder, and (d) this Agreement has been duly
executed and delivered by such Stockholder, constitutes the legal, valid and
binding obligation of such Stockholder, and is enforceable against such
Stockholder in accordance with its terms.

          2.   Agreement to Vote Shares.  Each of the Stockholders agrees that
               ------------------------                                       
during the term of this Agreement it will vote such Stockholder's Shares and any
New Shares 

                                      -2-

 
(as defined in Section 6 hereof), and will cause any holder of record
of such Shares or New Shares to vote such Stockholder's Shares and New Shares:
(a) in favor of the Merger Transactions and the issuance of shares of Parent
Common Stock in connection with the Merger at every meeting of the stockholders
of Parent at which such matters are considered and at every adjournment thereof,
and in connection with any written consent of the stockholders of Parent (b) in
favor of each of the Nominees (as defined in Section 6.11(c) of the Merger
Agreement) to Parent's Board of Directors, (c) against any action or agreement
that would compete with, impede, interfere with or attempt to discourage the
Merger Transactions, or inhibit the timely consummation of the Merger
Transactions, (d) against any action or agreement that would result in a breach
in any material respect of any covenant, representation or warranty or any other
obligation of Parent or Merger Sub under the Merger Agreement and (e) against
any merger, consolidation, business combination, reorganization,
recapitalization, liquidation or sale or transfer of any material assets of
Parent, Merger Sub or their respective subsidiaries that could compete with,
impede, interfere with or attempt to discourage the Merger Transactions or
inhibit the timely consummation of the Merger Transactions.  Each Stockholder
agrees to deliver to the Company upon request a proxy substantially in the form
attached hereto as Exhibit B, which proxy shall be irrevocable during the term
of this Agreement to the fullest extent permitted under Delaware law.

          3.   No Voting Trusts.  Each of the Stockholders agrees that they will
               ----------------                                                 
not, nor will they permit any entity under their control to, deposit any of
their Shares or any New Shares held by them in a voting trust or subject any of
their Shares or any New Shares held by them to any arrangement with respect to
the voting of such Shares or New Shares that would result in a stockholder's
vote or action by consent of the stockholders of Parent in opposition to or in
competition with the consummation of the Merger Transactions.

          4.   No Proxy Solicitations.  Each of the Stockholders agrees that
               ----------------------                                       
such Stockholder will not, nor will such Stockholder permit any entity under
such Stockholder's control to, (a) solicit proxies or become a "participant" in
a "solicitation" (as such terms are defined in Regulation 14A under the 1934
Act) in opposition to or competition with the consummation of the Merger
Transactions or otherwise encourage or assist any party in taking or planning
any action which would compete with, impede, interfere with or attempt to
discourage the Merger 

                                      -3-

 
Transactions or inhibit the timely consummation of the Merger Transactions, (b)
directly or indirectly encourage, initiate or cooperate in a stockholders' vote
or action by consent of Parent's stockholders in opposition to or in competition
with the consummation of the Merger Transactions, or (c) become a member of a
"group" (as such term is used in Section 13(d) of the 1934 Act) with respect to
any voting securities of Parent for the purpose of opposing or competing with
the consummation of the Merger Transactions.

          5.   Transfer and Encumbrance.  On or after the date hereof and during
               ------------------------                                         
the term of this Agreement, each of the Stockholders agrees not to transfer,
sell, offer, exchange, pledge or otherwise dispose of or encumber any of such
Stockholder's Shares or New Shares, unless the transferee agrees in written form
satisfactory to the Company to be bound by the terms of this Agreement.

          6.   Additional Purchases.  Each of the Stockholders agrees that such
               --------------------                                            
Stockholder will not purchase or otherwise acquire beneficial ownership of any
shares of Parent Common Stock, Preferred Shares, 7% Cumulative Convertible
Exchangeable Preferred Stock, par value $100 per share (the "7% Preferred
Stock") or any other capital stock of Parent after the execution of this
Agreement ("New Shares"), nor will any Stockholder voluntarily acquire the right
to vote or share in the voting of any shares of Parent Common Stock, Preferred
Shares, 7% Preferred Stock or any other capital stock of Parent other than the
Shares, unless such Stockholder agrees to deliver to the Company immediately
after such purchase or acquisition an irrevocable proxy in the form attached
hereto as Exhibit B with respect to such shares.  Each of the Stockholders also
severally agrees that any New Shares acquired or purchased by such Stockholder
shall be subject to the terms of this Agreement to the same extent as if they
constituted Shares.

          7.   Specific Performance.  Each party hereto acknowledges that it
               --------------------                                         
will be impossible to measure in money the damage to the other party if a party
hereto fails to comply with any of the obligations imposed by this Agreement,
that every such obligation is material and that, in the event of any such
failure, the other party will not have an adequate remedy at law or damages.
Accordingly, each party hereto agrees that injunctive relief or other equitable
remedy, in addition to remedies at law or damages, is the appropriate remedy for
any such failure and will not oppose the granting of such relief on the basis
that the other party has an adequate remedy at law.  Each party hereto agrees
that it will not seek, and agrees to waive any 

                                      -4-

 
requirement for, the securing or posting of a bond in connection with any other
party's seeking or obtaining such equitable relief.

          8.   Entire Agreement.  This Agreement supersedes all prior
               ----------------                                      
agreements, written or oral, among the parties hereto with respect to the
subject matter hereof and con  tains the entire agreement among the parties with
respect to the subject matter hereof.  This Agreement may not be amended,
supplemented or modified, and no provisions hereof may be modified or waived,
except by an instrument in writing signed by all the parties hereto.  No waiver
of any provisions hereof by any party shall be deemed a waiver of any other
provisions hereof by any such party, nor shall any such waiver be deemed a
continuing waiver of any provision hereof by such party.

          9.   Notices.  Any notice, request, instruction or other document to
               -------                                                        
be given hereunder by any party to the others shall be in writing and delivered
personally or sent by registered or certified mail, postage prepaid, or by
facsimile:

          if to any Stockholder:
          --------------------- 

          Daniel A. D'Aniello
          Managing Director
          The Carlyle Group
          1001 Pennsylvania Avenue, N.W.
          Suite 220 South
          Washington, DC 20004-2505

          if to Parent or Merger Sub:
          -------------------------- 

          James G. Kirk
          International Technology Corporation
          2790 Mosside Boulevard
          Monroeville, PA 15146-2792
          Telecopier:  (412) 858-3978

          with a copy to:

          Peter F. Ziegler
          Gibson, Dunn & Crutcher LLP
          333 South Grand Avenue
          Los Angeles, California 90071
          Telecopier:  (213) 229-7520

                                      -5-

 
          if to the Company:
          ----------------- 

          Steven E. Harbour
          OHM Corporation
          5445 Triangle Parkway, Suite 400
          Norcross, Georgia 30092

          with a copy to:

          Joseph B. Frumkin
          Sullivan & Cromwell
          125 Broad Street
          New York, New York 10004
          Telecopier:  (212) 558-3588

          and a copy to:

          Thomas C. Daniels
          Jones Day Reavis & Pogue
          North Point
          901 Lakeside Avenue
          Cleveland, Ohio 44114
          Telecopier:  (216) 579-0212

or to such other persons or addresses as may be designated in writing by the
party to receive such notice as provided above.

          10. Miscellaneous.
              ------------- 

          (a) This Agreement shall be deemed a contract made under, and for all
purposes shall be construed in accordance with, the laws of the State of
Delaware applicable to agreements executed in and solely to be performed within
such state.

          (b)  If any provision of this Agreement or the application of such
provision to any person or circumstances shall be held invalid or unenforceable
by a court of competent jurisdiction, such provision or application shall be
unenforceable only to the extent of such invalidity or unenforceability and the
remainder of the provision held invalid or unenforceable and the application of
such provision to persons or circumstances, other than the party as to which it
is held invalid, and the remainder of this Agreement, shall not be affected.

          (c)  This Agreement may be executed in one or more counterparts, each
of which shall be deemed to be an 

                                      -6-

 
original but all of which together shall constitute one and the same instrument.

          (d)  This Agreement shall terminate automatically upon the termination
of the Merger Agreement or upon the Effective Time of the Merger.  This
Agreement shall not otherwise be terminable.

          (e)  Each party hereto shall execute and deliver such additional
documents as may be necessary or desirable to effect the transactions
contemplated by this Agreement.

          (f)  All Section headings herein are for convenience of reference only
and are not part of this Agreement, and no construction or reference shall be
derived therefrom.

          (g) The obligations of the Stockholders set forth in this Agreement
shall not be effective or binding upon any Stockholder until after such time as
the Merger Agreement is executed and delivered by the Company, Parent and Merger
Sub, and the parties agree that there is not and has not been any other
agreement, arrangement or understanding between the parties hereto with respect
to the matters set forth herein.

                                      -7-

 
          IN WITNESS WHEREOF, the parties hereto have executed and delivered
this Agreement as of the date first written above.

                                     International Technology
                                     Corporation
                                       
                                       
                                       
                                     By:__________________________
                                        Name:
                                        Title:
                                       
                                       
                                       
                                     OHM Corporation
                                       
                                       
                                       
                                     By:__________________________
                                        Name:
                                        Title:
                                       
                                       
                                       
                                     Carlyle Partners II, L.P.
                                       
                                     By:   its General Partner, TC Group, L.L.C.
                                       
                                       
                                     By:__________________________
                                        Name:   Daniel A. D'Aniello
                                        Title:  Managing Director

                                      -8-

 
                                     Carlyle Partners III, L.P.
                                   
                                     By:   its General Partner, TC 
                                           Group, L.L.C.
                                   
                                   
                                     By:__________________________
                                        Name:   Daniel A. D'Aniello
                                        Title:  Managing Director
                                   
                                   
                                   
                                     Carlyle International Partners II, L.P.
                                   
                                     By:   its General Partner, TC 
                                           Group, L.L.C.
                                   
                                   
                                     By:__________________________
                                        Name:   Daniel A. D'Aniello
                                        Title:  Managing Director
                                   
                                   
                                   
                                     Carlyle International Partners III, L.P.
                                   
                                     By:   its General Partner, TC 
                                           Group, L.L.C.
                                   
                                   
                                     By:__________________________
                                        Name:   Daniel A. D'Aniello
                                        Title:  Managing Director
                                   
                                   
                                   
                                     C/S International Partners
                                   
                                     By:   its General Partner, TC 
                                           Group, L.L.C.
                                   
                                   
                                     By:__________________________
                                        Name:   Daniel A. D'Aniello
                                        Title:  Managing Director
                                   
                                   
                                   
                                     Carlyle Investment Group, L.P.
                                   
                                     By:   its General Partner, TC 
                                           Group, L.L.C.
                                   
                                   
                                     By:__________________________
                                        Name:   Daniel A. D'Aniello
                                        Title:  Managing Director

                                      -9-

 
                                    Carlyle-IT International Partners, L.P.
                                  
                                    By:   its General Partner, TC 
                                          Group, L.L.C.
                                  
                                  
                                    By:__________________________
                                       Name:   Daniel A. D'Aniello
                                       Title:  Managing Director
                                  
                                  
                                    Carlyle-IT International 
                                    Partners II, L.P.
                                  
                                    By:   its General Partner, TC 
                                          Group, L.L.C.
                                  
                                  
                                    By:__________________________
                                       Name:   Daniel A. D'Aniello
                                       Title:  Managing Director
                                  
                                  
                                  
                                    Carlyle-IT Partners, L.P.
                                  
                                    By:   its General Partner, TC 
                                          Group, L.L.C.
                                  
                                  
                                    By:__________________________
                                       Name:   Daniel A. D'Aniello
                                       Title:  Managing Director

                                      -10-

 
                              State Board of Administration of Florida separate
                              account maintained pursuant to an Investment
                              Management Agreement dated as of September 6, 1996
                              between the State Board of Administration of
                              Florida, Carlyle Investment Group, L.P. and
                              Carlyle Investment Management, L.L.C.

                              By:   Carlyle Investment 
                                    Management, L.L.C., as
                                    Investment Manager



                              By:   _________________________

                              Its:  _________________________

                                      -11-

 
                                                                     (EXHIBIT A)



                                            Number of Shares
                                       of Cumulative Convertible
        Name of Stockholder          Participating Preferred Stock
        -------------------          -----------------------------

Carlyle Partners II, L.P.                       1,781,965

Carlyle Partners III, L.P.                         81,357

Carlyle International Partners                  1,504,210
  II, L.P.                            

Carlyle International III, L.P.                    81,042

C/S International Partners                        338,682

Carlyle Investment Group, L.P.                      1,907

Carlyle-IT International                        2,366,299
  Partners, L.P.                      

Carlyle-IT International                           79,765
  Partners II, L.P.                   

The State Board of Administration                 748,520
  of the State of Florida             

Carlyle-IT Partners, L.P.                         195,107

 
                                                                     (EXHIBIT B)
                                 FORM OF PROXY


          The undersigned, for consideration received, hereby appoints [insert
names of Company designees] and each of them my proxies, with power of
substitution and resubstitution, to vote all shares of Cumulative Convertible
Participating Preferred Stock, par value $100 per share, of International
Technology Corporation, a Delaware corporation ("Parent"), [and [insert any
other Shares (as defined in the Voting Agreement) owned by Stockholder]] owned
by the undersigned at the Special Meeting of Stockholders of Parent to be held
[insert date, time and place] and at any adjournment thereof IN FAVOR OF
consummation of the Offer, the Merger and the other transactions (the "Merger
Transactions") contemplated by the Merger Agreement, dated as of January 15,
1998 (the "Merger Agreement"), among OHM Corporation (the "Company"), Parent and
IT-Ohio, Inc. ("Merger Sub"), IN FAVOR OF the issuance of shares of common stock
of Parent in connection with the merger of Merger Sub with and into the Company,
IN FAVOR OF each of the Nominees (as defined in Section 6.11(c) of the Merger
Agreement) to Parent's Board of Directors and AGAINST [insert description of any
action or agreement that would compete with, impede, interfere with or attempt
to discourage the Merger Transactions or inhibit the timely consummation of the
Merger Transactions or any action or agreement that would result in a breach in
any material respect of any covenant, representation or warranty or any other
obligation of Parent under the Merger Agreement or any merger, consolidation,
business combination, reorganization, recapitalization, liquidation or sale or
transfer of any material assets of Parent or its subsidiaries].  This proxy is
coupled with an interest, revokes all prior proxies granted by the undersigned
and is irrevocable until such time as the Voting Agreement, dated as of January
15, 1998 among certain stockholders of Parent, including the undersigned,
Parent, Merger Sub and the Company terminates in accordance with its terms.


                                               Dated ______, 1998
                                            
                                               [NAME OF STOCKHOLDER]
                                            
                                            
                                            
                                               By:___________________________