<PAGE>

                                                                 Exhibit 4.03(c)

                   OCCUPATIONAL HEALTH + REHABILITATION INC
                               175 Derby Street
                         Hingham, Massachusetts  02043



Mr. George H. Dixon, Director                     As of December 30, 1999
Fleet National Bank (f/k/a BankBoston, N.A.)
100 Federal Street
Boston, Massachusetts  02110

     RE:  REVOLVING CREDIT AGREEMENT
          --------------------------

Dear Mr. Dixon:

     Reference is made to that certain Revolving Credit Agreement, dated as of
November 13, 1997, as amended as of  December 30, 1998 (the "Credit Agreement"),
between Occupational Health + Rehabilitation Inc (the "Company") and Fleet
National Bank (f/k/a BankBoston, N.A.)  (the "Bank").  Capitalized terms used
and not otherwise defined herein shall have the same meanings herein as in the
Credit Agreement, in each case, as amended by this letter agreement.  This
letter agreement amends, restates and supersedes that certain letter agreement
dated December 30, 1999 between the Company and the Bank.

     The Company has requested that the Bank consent to the amendments to the
Credit Agreement provided below, which amendments would have the effect of (i)
extending the Credit Expiration Date to June 30, 2001, (ii) amending certain of
the negative covenants under the Credit Agreement, (iii) reducing the required
Minimum Net Worth for the fiscal year ending December 31, 1999, and (iv) adding
certain financial covenants to the Credit Agreement.

     (a)  Section 1.1 of the Credit Agreement is hereby amended by deleting the
date "December 31, 1999" (as extended by prior agreement to "March 31, 2000")
appearing therein and inserting in lieu thereof the date "June 30, 2001."

     (b)  Section 1.2 of the Credit Agreement is hereby amended by adding at the
end of subsection (a) thereof the following:

          The Company hereby agrees that notwithstanding any provision hereof to
          the contrary, the aggregate amount of  outstanding Company Revolving
          Credit Loans the proceeds of which are or are to be used to finance
          Eligible Acquisitions consumated subsequent to April 1, 2000
          shall not at any time exceed $500,000.
<PAGE>

                                                                 Exhibit 4.03(c)

     (c)  Effective December 31, 1999, Section 1.4(a)(ii) of the Credit
Agreement is hereby amended by deleting the figure "sixty-five one hundredths
percent (65/100%)" appearing therein and inserting in lieu thereof the figure
"seventy-five one-hundredths percent (75/100%)."

     (d)  Section 6.1(f) of the Credit Agreement is hereby amended by deleting
the proviso thereto in its entirety.  Section 6.1(h) of the Credit Agreement is
hereby amended by deleting the proviso thereto in its entirety.

     (e)  Section 6.4 of the Credit Agreement is hereby deleted in its entirety.

     (f)  Section 7.1 of the Credit Agreement is hereby amended by deleting the
figure "$10,000,000" appearing therein with respect to the fiscal year ending
December 31, 1999 and inserting in lieu thereof the figure "$6,000,000."

     (g)  Section 7 of the Credit Agreement is hereby further amended by
deleting Section 7.2 thereof and adding the following provisions thereto after
Section 7.1 thereof:

     (Section)7.2 Quick Ratio. The Company will not permit the Quick Ratio for
     any fiscal quarter ending on or prior to December 31, 1999 to be less than
     1.20 to 1.00.

     (Section)7.3 Minimum Net Income. The Company will not permit the Net Income
     for the quarterly fiscal period ending on June 30, 2000 and for each
     quarterly fiscal period thereafter to be less than $1.00.

     (Section)7.4 Minimum Adjusted Operating Cash Flow to Total Fixed Charges.
     The Company will not permit the ratio of Adjusted Operating Cash Flow for
     the four (4) consecutive fiscal quarters ending on the measuring dates set
     forth below to Total Fixed Charges to be less than the respective amounts
     indicated below:

<TABLE>
<CAPTION>

                                                    Minimum Ratio of Adjusted
                                                       Operating Cash Flow
            Fiscal Quarter Ending                    to Total Fixed Charges
----------------------------------------------  ---------------------------------
<S>                                             <C>

March 31, 2000                                                          1.00:1.00
----------------------------------------------  ---------------------------------

June 30, 2000                                                           1.00:1.00
----------------------------------------------  ---------------------------------

September 30, 2000                                                      1.05:1.00
----------------------------------------------  ---------------------------------

December 31, 2000                                                       1.15:1.00
----------------------------------------------  ---------------------------------

March 31, 2001                                                          1.25:1.00
----------------------------------------------  ---------------------------------

June 30, 2001                                                           1.25:1.00
----------------------------------------------  ---------------------------------
</TABLE>

                                       2
<PAGE>

                                                                 Exhibit 4.03(c)

     (Section)7.5 Maximum Adjusted Total Senior Funded Debt to EBITDAR. The
     Company will not permit the ratio of Adjusted Total Senior Funded Debt as
     of the last day of any fiscal quarter set forth below to EBITDAR for any
     period of four (4) consecutive fiscal quarters ending on such measuring
     dates to be greater than the respective amounts indicated below:

<TABLE>
<CAPTION>

                                                 Maximum Ratio of Adjusted Total
            Fiscal Quarter Ending                 Senior Funded Debt to EBITDAR
---------------------------------------------------------------------------------
<S>                                             <C>

March 31, 2000                                                          6.25:1.00
---------------------------------------------------------------------------------

June 30, 2000                                                           6.25:1.00
---------------------------------------------------------------------------------

September 30, 2000                                                      5.50:1.00
---------------------------------------------------------------------------------

December 31, 2000                                                       5.00:1.00
---------------------------------------------------------------------------------

March 31, 2001                                                          4.75:1.00
---------------------------------------------------------------------------------

June 30, 2001                                                           4.50:1.00
---------------------------------------------------------------------------------
</TABLE>

     (i)  Section 8.1 of the Credit Agreement is hereby amended by adding after
subsection (l) thereof the following: "or (m) the Company shall have failed to
cause the holders of the outstanding redeemable, convertible preferred stock of
the Company to have executed and delivered to the Bank, on or prior to June 30,
2000, a subordination agreement reasonably satisfactory to the Company and the
Bank pursuant to which the repayment of all Indebtedness of the Company in
respect of such stock shall have been subordinated to the Indebtedness of the
Company evidenced by the Company Revolving Credit Note, the Indebtedness
evidenced by the JV Revolving Credit Agreements, and the Indebtedness of the
Company outstanding on April 1, 2000 under that certain Master Lease Financing
Agreement, dated as of September 15, 1998, as amended, between the Company and
BancBoston Leasing Inc."

     (j)  Section 9 of the Credit Agreement is hereby amended by deleting the
definitions  "EBITDA" and "Eligible Acquisition" set forth therein and by adding
in the appropriate places therein the following definitions:

     Adjusted Operating Cash Flow:  for any period, EBITDA for such period,
     after (a) restoring thereto amounts deducted for rental and operating lease
     expense paid or payable and (b) deducting amounts paid for capital
     expenditures and acquisitions that were not funded by financing obtained
     from an external source.

                                       3
<PAGE>

                                                                 Exhibit 4.03(c)

     Adjusted Total Senior Funded Debt: on the applicable measuring date, the
     sum of (a) the aggregate amount outstanding (determined in accordance with
     generally accepted accounting principles on a consolidated basis) of every
     obligation for senior unsubordinated interest-bearing debt, including the
     outstanding balance of the Revolving Credit Loans, Capital Leases, and
     promissory notes of the Company and its Subsidiaries, and (b) the product
     of eight (8) times the rental and operating lease expense paid or payable
     in any period of four (4) fiscal quarters ending on the measuring date.

     EBITDA:  for any period, Net Income for such period, after restoring
     thereto amounts deducted for (a) depreciation and amortization, (b)
     Interest Expense, (c) income taxes in respect of income and profits, and
     (d) minority interest in net profit of subsidiaries.

     EBITDAR:  for any period, the sum of (a) EBITDA for such period and (b) the
     rental and operating lease expense paid or payable in such period.

     Eligible Acquisition:  shall mean the acquisition by the Company of the
     assets or Shares of any Person engaged in the business of providing
     occupational health or other medical services or other related business or
     activities incidental thereto, provided that (i) the aggregate amount
     representing the purchase price for all Eligible Acquisitions (including
     any Indebtedness assumed in connection therewith other than routine
     operating lease, trade debt and other similar ordinary course of business
     obligations) for the period from April 1, 2000 through the Credit
     Expiration Date shall not exceed $500,000 without the prior written consent
     of the Bank, which consent shall not unreasonably withheld, conditioned or
     delayed, (ii) prior to any such Eligible Acquisition, and after giving
     effect thereto (including any Company Revolving Credit Loans requested by
     the Company in connection with such Eligible Acquisitions), there shall
     exist no Company Default or Company Event of Default, and (iii) prior to
     any such Eligible Acquisition, the Company shall have provided to the Bank
     notice of such proposed acquisition and an information package regarding
     such proposed acquisition which shall contain a description of the assets
     to be acquired, the purchase price and terms of payment for such assets and
     such other information regarding such proposed acquisition as the Bank may
     reasonably request at least ten (10) days prior to the proposed closing of
     such proposed acquisition.  If such proposed acquisition involves the
     acquisition of Shares of a Person, such notice shall confirm that the
     Company shall own, directly or indirectly, a majority of the equity
     interests in the acquired Person and shall control a majority of any voting
     securities and/or shall otherwise control the governance of the acquired
     Person.  In connection with such proposed acquisition, if such proposed
     acquisition involves the acquisition of Shares of a Person, the Company
     shall have granted to the Bank first priority security interests in the
     Company"s equity interest in such acquired Person by means of a pledge.
     Any assets

                                       4
<PAGE>

                                                                 Exhibit 4.02(c)

     acquired in connection with such proposed acquisition shall constitute
     after-acquired property subject to the Bank"s existing security interests
     under the Company Security Documents unless such assets are simultaneously
     contributed to a joint venture, which is majority owned by the Company.

     Net Income:  net income or net loss of the Company and its Subsidiaries for
     the period in question, determined in accordance with generally accepted
     accounting principles on a consolidated basis, provided that such net
     income or net loss as determined for any period which includes the fiscal
     quarter ending December 31, 1999 shall exclude those restructuring charges
     and the write-off of the $292,000 Community Rehabilitation Center note
     receivable effected by the Company during the fiscal quarter ending
     December 31, 1999.

     Total Fixed Charges:   the aggregate amount (determined in accordance with
     generally accepted accounting principles on a consolidated basis) for the
     prior four (4) consecutive fiscal quarters ended as of the measuring date
     of (a) long-term debt (excluding, if applicable, the outstanding balance of
     the Revolving Credit Loans) and Capital Lease payments made or required to
     be paid and dividends in respect of the Company's outstanding redeemable,
     convertible preferred stock paid or payable, (b) Interest Expense, and (c)
     rental and operating lease expense paid or payable.

     (k)  Section 1.1 of Exhibit A to the Credit Agreement (the form of JV
Revolving Credit Agreement) is hereby amended by deleting the date "December 31,
1999" (as extended by prior agreement to "March 31, 2000") appearing therein and
inserting in lieu thereof the date "June 30, 2001".  The Bank hereby agrees that
it shall negotiate with each JV Borrower (as defined in the JV Revolving Credit
Agreement) an option to have outstanding loans pursuant to the JV Revolving
Credit Agreement with such terms and conditions as shall be mutually acceptable
to the Bank and such JV Borrower with the interest rate based on a money market
option if the JV Borrower has an equity owner (or affiliate) that is publicly
rated BBB+ or better and that jointly and severally guarantees the repayment in
full of the credit extended.  The form of the JV Borrower Revolving Credit Note
(as defined in the JV Revolving Credit Agreement) is hereby amended by deleting
the date "December 31, 1999" (as extended by prior agreement to "March 31,
2000") appearing in the first paragraph thereof and inserting in lieu thereof
the date "June 30, 2001."

     Except as amended hereby, the Credit Agreement and each of the other
Company Loan Documents shall remain in full force and effect.  Concurrently with
the execution and delivery of this letter, the Company shall execute and deliver
to the Bank a replacement Company  Revolving Credit Note in the form attached
hereto as Exhibit A, and the Company shall pay to the Bank a fee in the amount
of $7,500, in consideration of the Bank"s agreements herein and shall pay the
attorneys" fees of counsel to the Bank in connection with this letter.  In
addition, the

                                       5
<PAGE>

                                                                 Exhibit 4.03(c)

Company is delivering herewith the budget for the Company and its Subsidiaries
for the fiscal year ending December 31, 2000 and the projections for the six-
month period ending June 30, 2001. On or prior to April 10, 2000, the Company
and the Bank shall prepare covenant compliance schedules incorporating the
financial covenants provided above and financial methodology (including, without
limitation, with respect to rental and operating expense paid or payable)
consistent with the Company's financial statements.

                                       6
<PAGE>

                                                                 Exhibit 4.03(c)

     If you are in agreement with the foregoing, please execute each of the
accompanying counterparts to this letter, whereupon this letter shall become a
binding agreement under seal between the Company and the Bank.  Please then
return one of such counterparts to the Company.


                              Very truly yours,


                              OCCUPATIONAL HEALTH +
                               REHABILITATION INC


                              By:/s/ John C. Garbarino
                                 ---------------------
                                 President and Chief Executive Officer

ACKNOWLEDGED AND AGREED:
-----------------------

FLEET NATIONAL BANK (f/k/a BankBoston, N.A.)



By:/s/ George H. Dixon
   -------------------
  Director

                                       7