<PAGE>

                                                                EXHIBIT 4.7.1(k)

                                                      UPON RECORDING, RETURN TO:
                                                            MS. SHAWNE M. KEENAN
                                                 SUTHERLAND ASBILL & BRENNAN LLP
                                                      999 PEACHTREE STREET, N.E.
                                                     ATLANTA, GEORGIA 30309-3996

   PURSUANT TO SECTION 44-14-35.1 OF OFFICIAL CODE OF GEORGIA ANNOTATED, THIS
    INSTRUMENT EMBRACES, COVERS AND CONVEYS SECURITY TITLE TO AFTER-ACQUIRED
                            PROPERTY OF THE GRANTOR

================================================================================
================================================================================

                          OGLETHORPE POWER CORPORATION
                      (AN ELECTRIC MEMBERSHIP CORPORATION),
                                    GRANTOR,

                                       to

                             SUNTRUST BANK, ATLANTA,
                                     TRUSTEE

                               TENTH SUPPLEMENTAL
                                    INDENTURE

                                 Relating to the
                             Series 1999 Lease Notes

                          Dated as of December 1, 1999

                           FIRST MORTGAGE OBLIGATIONS

================================================================================
================================================================================

<PAGE>


        THIS TENTH SUPPLEMENTAL INDENTURE, dated as of December 1, 1999, is
between OGLETHORPE POWER CORPORATION (AN ELECTRIC MEMBERSHIP CORPORATION), an
electric membership corporation organized and existing under the laws of the
State of Georgia, as Grantor (hereinafter called the "Company"), and SUNTRUST
BANK, ATLANTA, a banking corporation organized and existing under the laws of
the State of Georgia, as Trustee (in such capacity, the "Trustee").

        WHEREAS, the Company has heretofore executed and delivered to the
Trustee an Indenture, dated as of March 1, 1997 (hereinafter called the
"Original Indenture") for the purpose of securing its Existing Obligations and
providing for the authentication and delivery of Additional Obligations by the
Trustee from time to time under the Original Indenture;

        WHEREAS, the Company has entered into a Participation Agreement, dated
as of December 29, 1999 (the "Participation Agreement"), among itself, First
Security Bank, National Association, a national banking organization, as Owner
Trustee, Pitney Bowes Credit Corporation, a Delaware corporation, as Trustor,
Black Diamond Energy, Inc., a Georgia corporation, as Seller, and CoBank, ACB,
as Lender, pursuant to which the parties are engaging in a transaction involving
the lease by the Owner Trustee of certain open top hopper railcars to the
Company (capitalized terms used herein and not otherwise defined herein shall
have the meanings ascribed to them in the Original Indenture as provided in
Section 2.1 hereof, or if not defined in the Original Indenture, shall have the
meanings ascribed to them in the Glossary attached as Exhibit B to the
Participation Agreement, except, in cases where the context clearly indicates
otherwise);

        WHEREAS, pursuant to Section 3.2(b) of the Participation Agreement, the
Company is required to issue Series 1999 Lease Notes at each closing to support
certain of the Company's obligations under the Lease to pay Casualty Loss Value
and Make Whole Premium Amount upon an Event of Default under the Lease;

        WHEREAS, the Company desires to execute and deliver this Tenth
Supplemental Indenture, in accordance with the provisions of the Original
Indenture, for the purpose of providing for the creation and designation of the
Series 1999 Lease Notes as Additional Obligations and specifying the form and
provisions of the Series 1999 Lease Notes (the Original Indenture, as
heretofore, hereby and hereafter supplemented and modified, being herein
sometimes called the "Indenture");

        WHEREAS, the Series 1999 Lease Notes are to be authenticated and
delivered pursuant to Section 4.3 of the Original Indenture upon the basis of
retirement of Obligations;

        WHEREAS, Section 12.1 of the Original Indenture provides that, without
the consent of the Holders of any of the Obligations at the time Outstanding,
the Company, when authorized by a Board Resolution, and the Trustee, may enter
into supplemental indentures for the purposes and subject to the conditions set
forth in said Section 12.1; and

        WHEREAS, all acts and proceedings required by law and by the Articles of
Incorporation and Bylaws of the Company necessary to secure under the Indenture
the payment of the principal of (and

<PAGE>

premium, if any) and interest on the Series 1999 Lease Notes, to make the Series
1999 Lease Notes to be issued hereunder, when executed by the Company,
authenticated and delivered by the Trustee and duly issued, the valid, binding
and legal obligations of the Company, and to constitute the Indenture a valid
and binding lien for the security of the Series 1999 Lease Notes, in accordance
with its terms, have been done and taken; and the execution and delivery of this
Tenth Supplemental Indenture has been in all respects duly authorized;

        NOW, THEREFORE, THIS TENTH SUPPLEMENTAL INDENTURE WITNESSES, that,
to secure the payment of the principal of (and premium, if any) and interest on
the Outstanding Secured Obligations, including, when issued, the Series 1999
Lease Notes, to confirm the lien of the Indenture upon the Trust Estate,
including property purchased, constructed or otherwise acquired by the Company
since the date of execution of the Original Indenture, to secure performance of
the covenants therein and herein contained, to declare the terms and conditions
on which the Series 1999 Lease Notes are secured, and in consideration of the
premises thereof and hereof, the Company by these presents does grant, bargain,
sell, alienate, remise, release, convey, assign, transfer, mortgage,
hypothecate, pledge, set over and confirm to the Trustee, and its successors and
assigns in the trust created thereby and hereby in trust, all property, rights,
privileges and franchises (other than Excepted Property or Excludable Property)
of the Company of the character described in the Granting Clauses of the
Original Indenture, including all such property, rights, privileges and
franchises acquired since the date of execution of the Original Indenture,
subject to all exceptions, reservations and matters of the character referred to
in the Indenture, and does grant a security interest therein for the purposes
expressed herein and in the Original Indenture subject in all cases to Sections
5.2 and 11.2 B of the Original Indenture and to the rights of the Company under
the Original Indenture, including the rights set forth in Article V thereof, but
expressly excepting and excluding from the lien and operation of the Indenture
all properties of the character specifically excepted as "Excepted Property" or
"Excludable Property" in the Original Indenture to the extent contemplated
thereby.

        PROVIDED, HOWEVER, that if, upon the occurrence of an Event of Default,
the Trustee, or any separate trustee or co-trustee appointed under Section 9.14
of the Original Indenture or any receiver appointed pursuant to statutory
provision or order of court, shall have entered into possession of all or
substantially all of the Trust Estate, all the Excepted Property described or
referred to in Paragraphs A through H, inclusive, of "Excepted Property" in the
Original Indenture then owned or thereafter acquired by the Company, shall
immediately, and, in the case of any Excepted Property described or referred to
in Paragraphs I, J, L, N and P of "Excepted Property" in the Original Indenture
(excluding the property described in Section 2 of EXHIBIT B in the Original
Indenture), upon demand of the Trustee or such other trustee or receiver, become
subject to the lien of the Indenture to the extent permitted by law, and the
Trustee or such other trustee or receiver may, to the extent permitted by law,
at the same time likewise take possession thereof, and whenever all Events of
Default shall have been cured and the possession of all or substantially all of
the Trust Estate shall have been restored to the Company, such Excepted Property
shall again be excepted and excluded from the lien of the Indenture to the
extent and otherwise as hereinabove set forth and as set forth in the Original
Indenture.

        The Company may, however, pursuant to the Granting Clause Third of the
Original Indenture, subject to the lien of the Indenture any Excepted Property
or Excludable Property, whereupon the same shall cease to be Excepted Property
or Excludable Property.

                                        2

<PAGE>


        TO HAVE AND TO HOLD all such property, rights, privileges and franchises
hereby and hereafter (by Supplemental Indenture or otherwise) granted,
bargained, sold, alienated, remised, released, conveyed, assigned, transferred,
mortgaged, hypothecated, pledged, set over or confirmed as aforesaid, or
intended, agreed or covenanted so to be, together with all the tenements,
hereditaments and appurtenances thereto appertaining (said properties, rights,
privileges and franchises, including any cash and securities hereafter deposited
or required to be deposited with the Trustee (other than any such cash which is
specifically stated in the Indenture not to be deemed part of the Trust Estate)
being part of the Trust Estate), unto the Trustee, and its successors and
assigns in the trust herein created, forever.

        SUBJECT, HOWEVER, to (i) Permitted Exceptions and (ii) to the extent
permitted by Section 13.6 of the Original Indenture as to property hereafter
acquired (a) any duly recorded or perfected prior mortgage or other lien that
may exist thereon at the date of the acquisition thereof by the Company and (b)
purchase money mortgages, other purchase money liens, chattel mortgages,
conditional sales agreements or other title retention agreements created by the
Company at the time of acquisition thereof.

        BUT IN TRUST, NEVERTHELESS, with power of sale, for the equal and
proportionate benefit and security of the Holders from time to time of all the
Outstanding Secured Obligations without any priority of any such Obligation over
any other such Obligation and for the enforcement of the payment of such
Obligations in accordance with their terms.

        UPON CONDITION that, until the happening of an Event of Default and
subject to the provisions of Article V of the Original Indenture, and not in
limitation of the rights elsewhere provided in the Original Indenture, including
the rights set forth in Article V of the Original Indenture, the Company shall
be permitted to (i) possess and use the Trust Estate, except cash, securities,
Designated Qualifying Securities and other personal property deposited, or
required to be deposited, with the Trustee, (ii) explore for, mine, extract,
separate and dispose of coal, ore, gas, oil and other minerals, and harvest
standing timber, and (iii) receive and use the rents, issues, profits, revenues
and other income, products and proceeds of the Trust Estate.

        THE INDENTURE, INCLUDING THIS TENTH SUPPLEMENTAL INDENTURE, is
intended to operate and is to be construed as a deed passing title to the Trust
Estate and is made under the provisions of the existing laws of the State of
Georgia relating to deeds to secure debt, and not as a mortgage or deed of
trust, and is given to secure the Outstanding Secured Obligations. Should the
indebtedness secured by the Indenture be paid according to the tenor and effect
thereof when the same shall become due and payable and should the Company
perform all covenants herein contained in a timely manner, then the Indenture
shall be canceled and surrendered.

        AND IT IS HEREBY COVENANTED AND DECLARED that the Series 1999 Lease
Notes are to be authenticated and delivered and the Trust Estate is to be held
and applied by the Trustee, subject to the covenants, conditions and trusts set
forth herein and in the Original Indenture, and the Company does hereby covenant
and agree to and with the Trustee, for the equal and proportionate benefit of
all Holders of the Outstanding Secured Obligations, as follows:

                                        3


<PAGE>


                                    ARTICLE I

                         THE SERIES 1999 LEASE NOTES AND
                       CERTAIN PROVISIONS RELATING THERETO

        SECTION 1.1   AUTHORIZATION AND TERMS OF THE SERIES 1999 LEASE NOTE.

        There shall be created and established a series of Additional Obligation
in the form of promissory notes known as and entitled the "Series 1999 Lease
Notes" (hereinafter referred to as the "Series 1999 Lease Notes"), the form,
terms and conditions of which shall be substantially as set forth in this
Section and Section 1.2.

        The Series 1999 Lease Notes shall be issuable in fully registered form
without coupons and in denominations necessary to issue Series 1999 Lease Notes
in the principal amounts required pursuant to Section 3 of the Participation
Agreement; PROVIDED that the aggregate principal amount payable at any one time
under the Series 1999 Lease Notes shall not exceed $13,500,000. Each Series 1999
Lease Note shall be dated the date of its authentication.

        For purposes of (i) receiving payment upon any Series 1999 Series Lease
Note, whether when due pursuant to the terms thereof or if the principal of all
Obligations is declared immediately due and payable following an Event of
Default as provided in Section 8.1 of the Indenture, or (ii) computing the
principal amount of Obligations held by any Holder of such Series 1999 Lease
Note in giving any request, demand, authorization, direction, notice, consent,
waiver or other action provided by the Indenture to be given or taken by
Holders, the principal amount of such Series 1999 Lease Note shall be the actual
Secured Amount then due and payable on the Series 1999 Lease Note at such time.
For the purposes of serving as the basis for authenticating and delivering
Additional Obligations under Section 4.3 of the Original Indenture the principal
amount of any Series 1999 Lease Note shall be the highest Secured Amount set
forth on Annex A to such Series 1999 Lease Note.

        Except in the event that the principal of the Series 1999 Lease Notes
shall not be paid when due or shall be declared due and payable in the manner
and with the effect provided in Section 8.2 of the Original Indenture, the
Series 1999 Lease Notes shall not bear interest. In the event that the principal
of the Series 1999 Lease Notes shall not be paid when due or shall be declared
due and payable in the manner and with the effect provided in Section 8.2 of the
Original Indenture, the Series 1999 Lease Notes shall bear interest on the
principal amount thereof from the date due or the date of such declaration, as
the case may be, at the Overdue Rate. Any such interest shall be payable
concurrently with the payment of the principal of such Series 1999 Lease Notes.

        Each Series 1999 Lease Note shall expire and be deemed to have been
cancelled upon the earlier of (i) the expiration or early termination of the
Lease with respect to all Units listed on the applicable Lease Supplement and
(x) if and to the extent required pursuant thereto, the return of such Units to
the Lessor pursuant to the terms and provisions of the Lease and (y) the making
of all payments required to be made by the Company in connection with the
expiration or early termination of the Lease with respect to such Units, and
(ii) payment of all amounts then due and payable on this Series 1999 Lease Note
following the Secured Amount becoming due and payable thereunder.


                                        4

<PAGE>


        All payments made on the Series 1999 Lease Notes shall be made in lawful
money of the United States of America to such address as the Holder thereof
shall direct, from time to time, by written notice to the Company and the
Trustee.

        SECTION 1.2   FORM OF THE SERIES 1999 LEASE NOTES.

        The Series 1999 Lease Notes, including the Trustee's authentication
certificate to be executed on such Notes, shall be substantially in the form of
EXHIBIT A attached hereto, with such appropriate insertions, omissions,
substitutions and other variations as are required or permitted in the Original
Indenture. The Series 1999A Lease Notes shall be numbered from 1 consecutively
upwards in order of issuance hereunder or shall have such other designation and
numbers as the Company may determine.

        SECTION 1.3 DESIGNATION OF AMBAC ASSURANCE CORPORATION AS CREDIT
ENHANCER FOR THE SERIES 1999 LEASE NOTES.

        In accordance with the provisions of the Original Indenture, Ambac
Assurance Corporation is hereby designated as the "Credit Enhancer" for the
Series 1999 Lease Notes, as such term is defined in Section 1.1 of the Original
Indenture.

                                   ARTICLE II

                                  MISCELLANEOUS

        SECTION 2.1 This Tenth Supplemental Indenture is executed and shall be
construed as an indenture supplemental to the Original Indenture, and shall form
a part thereof, and the Indenture, as heretofore supplemented and as hereby
supplemented and modified, is hereby confirmed. Except to the extent
inconsistent with the express terms hereof, all of the provisions, terms,
covenants and conditions of the Indenture shall be applicable to the Series 1999
Lease Notes to the same extent as if specifically set forth herein. All
references herein to Sections, definitions or other provisions of the Original
Indenture shall be to such Sections, definitions and other provisions as they
may be amended or modified from time to time pursuant to the Indenture. All
capitalized terms used in this Tenth Supplemental Indenture and not otherwise
defined herein shall have the same meanings ascribed to them in the Original
Indenture, or if not defined in the Original Indenture, the meanings ascribed to
them in the Glossary attached as Exhibit B to the Participation Agreement,
except, in cases where the context clearly indicates otherwise.

        SECTION 2.2 All recitals in this Tenth Supplemental Indenture are made
by the Company only and not by the Trustee; and all of the provisions contained
in the Original Indenture, in respect of the rights, privileges, immunities,
powers and duties of the Trustee shall be applicable in respect hereof as fully
and with like effect as if set forth herein in full.

        SECTION 2.3 Whenever in this Tenth Supplemental Indenture any of the
parties hereto is named or referred to, this shall, subject to the provisions of
Articles IX and XI of the Original Indenture, be deemed to include the
successors and assigns of such party, and all the covenants and Agreements in
this Tenth Supplemental Indenture contained by or on behalf of the Company, or
by or on behalf of

                                        5


<PAGE>


the Trustee shall, subject as aforesaid, bind and inure to the respective
benefits of the respective successors and assigns of such parties, whether so
expressed or not.

        SECTION 2.4 Nothing in this Tenth Supplemental Indenture, expressed or
implied, is intended, or shall be construed, to confer upon, or to give to, any
person, firm or corporation, other than the parties hereto and the Holders of
the Outstanding Secured Obligations, any right, remedy or claim under or by
reason of this Tenth Supplemental Indenture or any covenant, condition,
stipulation, promise or agreement hereof, and all the covenants, conditions,
stipulations, promises and agreements in this Tenth Supplemental Indenture
contained by or on behalf of the Company shall be for the sole and exclusive
benefit of the parties hereto, and of the Holders of Outstanding Secured
Obligations.

        SECTION 2.5 This Tenth Supplemental Indenture may be executed in several
counterparts, each of such counterparts shall for all purposes be deemed to be
an original, and all such counterparts, or as many of them as the Company and
the Trustee shall preserve undestroyed, shall together constitute but one and
the same instrument.

        SECTION 2.6 To the extent permitted by applicable law, this Tenth
Supplemental Indenture shall be deemed to be a Security Agreement and Financing
Statement whereby the Company grants to the Trustee a security interest in all
of the Trust Estate that is personal property or fixtures under the Uniform
Commercial Code, as adopted or hereafter adopted in one or more of the states in
which any part of the properties of the Company are situated. The mailing
address of the Company,

as debtor is:                2100 East Exchange Place
                             P.0. Box 1349
                             Tucker, Georgia 30085-1349,

and the mailing address of the Trustees, as secured party is:

                             SunTrust Bank, Atlanta
                             25 Park Place
                             Atlanta, Georgia 30303-2900

                                  [Signatures on Next Page.]


                                        6


<PAGE>


        IN WITNESS WHEREOF, the parties hereto have caused this Tenth
Supplemental Indenture to be duly executed under seal as of the day and year
first written above.

COMPANY:                              OGLETHORPE POWER
                                      CORPORATION (AN ELECTRIC MEMBERSHIP
                                      CORPORATION), an electric membership
                                      corporation organized under the laws
                                      of the State of Georgia

2100 East Exchange Place
P.O. Box 1349
Tucker, Georgia 30085-1349

                                      By: /s/ THOMAS A. SMITH
                                          -----------------------------------
                                          Thomas A. Smith
                                          President and Chief Executive Officer

Signed, sealed and delivered          Attest: /s/ PATRICIA N. NASH
by the Company in the presence of          ----------------------------------
                                           Patricia N. Nash
                                           Secretary

/s/ LYNN H. LESTER
----------------------------------
Witness

/s/ MELONEY A. CHENENEY
----------------------------------
Notary Public


(Notarial Seal)

My commission expires:   OCTOBER 20, 2000
                      -----------------------


                       [Signatures Continued on Next Page]


                                        7


<PAGE>



                   [Signatures Continued from Previous Page.]

TRUSTEE:                            SUNTRUST BANK, ATLANTA,
                                    a banking corporation organized and existing
                                    under the laws of the State of Georgia

Signed, sealed and delivered
by the trustee in the               By: /s/ BARTON A. DONALDSON
presence of:                            ----------------------------------
                                        Barton A. Donaldson
                                        Vice President

/s/ CADA T. KILGORE
--------------------------------
Witness

/s/ TERESA R. TURNER
--------------------------------
Notary Public

(Notarial Seal)                                    [BANK SEAL]

My commission expires:       APRIL 3, 2001
                       ----------------------


                                        8


<PAGE>


                                    EXHIBIT A

                        [Form of Series 1999 Lease Note]

Financial Guaranty Insurance Policy No. [ ] (the "Policy") with respect to
payments due for principal of and interest on this note has been issued by Ambac
Assurance Corporation ("Ambac Assurance"). All payments required to be made
under the Policy shall be made in accordance with the provisions thereof. The
owner of this note acknowledges and consents to the subrogation rights of Ambac
Assurance as more fully set forth in the Policy.

    THIS SERIES 1999 LEASE NOTE HAS NOT BEEN AND WILL NOT BE REGISTERED UNDER
      THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD, OFFERED
             FOR SALE OR OTHERWISE TRANSFERRED WITHOUT REGISTRATION
                UNDER SUCH ACT OR IN RELIANCE UPON AN APPLICABLE
                   EXEMPTION FROM REGISTRATION UNDER SUCH ACT.

                          OGLETHORPE POWER CORPORATION
                      (AN ELECTRIC MEMBERSHIP CORPORATION)

                             SERIES 1999 LEASE NOTE

No.                                                                        DATE:

     OGLETHORPE POWER CORPORATION (AN ELECTRIC MEMBERSHIP CORPORATION) (the
"Company"), an electric membership corporation organized and existing under the
laws of the State of Georgia, for value received hereby promises to pay to
[_______________] or its registered assigns, as principal due and payable
hereunder an amount equal to the [Casualty Loss Value] [Make Whole Premium
Amount] payable under Section [18.1.][18.2.] of the Equipment Lease Agreement,
dated as of December 29, 1999 (the "Lease"), between the Company and First
Security Bank, National Association, a national banking association, not in its
individual capacity, but solely as trustee under the Owner Trust Agreement,
dated as of December 29, 1999, with Pitney Bowes Credit Corporation (in such
capacity, the "Owner Trustee") for [the Loan Certificates relating to] the Units
listed on Schedule 1 to Lease Supplement No. [__] thereto [(the "Casualty Loss
Value")][(the "Make Whole Premium Amount")]; PROVIDED, HOWEVER, in no event
shall the principal amount payable hereunder (the "Secured Amount") exceed the
amount set forth on Annex A hereto for the date [Casualty Loss Value][the Make
Whole Premium Amount] becomes due and payable under Section [18.1][18.2] of the
Lease. The Secured Amount shall be due and payable at any time after [the
earlier of (i) the date Lender transfers the Loan Certificates to Trustor or
Lessor pursuant to Section 2.13 of the Loan Agreement or (ii) the date] the
Casualty Loss Value becomes due and payable pursuant to Section 18.1. of the
Lease [or is declared due and payable pursuant to the clause (ix) of Section
18.1.(b)] of the Lease, unless[, in either case,] payment of [Casualty Loss
Value][the Make Whole Premium Amount] is made in full under the Lease prior to
such time. In the event the Secured Amount becomes payable as provided in the
foregoing sentence, the Secured Amount shall bear interest at the Overdue Rate
(as

                                       A-1


<PAGE>

defined in or pursuant to the Lease) from the date when payable to the date of
payment by the Company hereunder or under the Lease.

        For purposes of (i) receiving payment upon this Series 1999 Lease Note,
whether at when due pursuant to the preceding paragraph or if the principal of
all Obligations is declared immediately due and payable following an Event of
Default as provided in Section 8.1 of the Indenture, or (ii) computing the
principal amount of Obligations held by any Holder of this Series 1999 Lease
Note in giving any request, demand, authorization, direction, notice, consent,
waiver or other action provided by the Indenture to be given or taken by
Holders, the principal amount of this Series 1999 Lease Note shall be the actual
Secured Amount then due and payable on this Series 1999 Lease Note at such time.
For the purposes of serving as the basis for authenticating and delivering
Additional Obligations under Section 4.3 of the Original Indenture the principal
amount of this Series 1999 Lease Note shall be $[______] [fill in the highest
amount set forth on Annex A].

        Except in the event that the principal of this Series 1999 Lease Note
shall not be paid when due or shall be declared due and payable in the manner
and with the effect provided in Section 8.2 of the Indenture, this Series 1999
Lease Note shall not bear interest. In the event that the principal of this
Series 1999 Lease Note shall not be paid when due or shall be declared due and
payable in the manner and with the effect provided in Section 8.2 of the
Original Indenture, this Series 1999 Lease Note shall bear interest on the
principal amount thereof from the date due or the date of such declaration, as
the case may be, at the Overdue Rate. Any such interest shall be payable
concurrently with the payment of the principal of this Series 1999 Lease Note.

        This Series 1999 Lease Note is a duly authorized obligation of the
Company issued under and equally and ratably secured by the Indenture, dated as
of March 1, 1997 (the "Original Indenture"), between the Company, as grantor,
and SunTrust Bank, Atlanta, as trustee (in such capacity, the "Indenture
Trustee"), as supplemented by the First Supplemental Indenture, dated as of
October 1, 1997, the Second Supplemental Indenture, dated as of January 1, 1998,
the Third Supplemental Indenture, dated as of January 1, 1998, the Fourth
Supplemental Indenture, dated as of March 1, 1998, the Fifth Supplemental
Indenture, dated as of April 1, 1998, the Sixth Supplemental Indenture, dated as
of January 1, 1999, the Seventh Supplemental Indenture, dated as of January 1,
1999, the Eighth Supplemental Indenture, dated as of November 1, 1999, the Ninth
Supplemental Indenture, dated as of November 1, 1999, and the Tenth Supplemental
Indenture, dated as of December 1, 1999 (the "Tenth Supplemental Indenture")
between the Company and the Indenture Trustee (the Original Indenture, as
supplemented, the "Indenture"). Reference is hereby made to the Indenture for a
statement of the description of the properties thereby mortgaged, pledged and
assigned, the nature and extent of the security and the respective rights,
limitations of rights, duties and immunities thereunder of the Company, the
Indenture Trustee and the holder of this Series 1999 Lease Note and of the terms
upon which this Series 1999 Lease Note is authenticated and delivered. This
Series 1999 Lease Note is created by the Tenth Supplemental Indenture and
designated as the "Series 1999 Lease Note."

        All payments hereon are to be made in lawful money of the United States
of America to such address as the Holder of this Series 1999 Lease Note shall
direct, from time to time, by written notice to the Company and the Trustee.

                                       A-2


<PAGE>


        No recourse shall be had for the payments required hereby or for any
claim based herein or in the Lease or in the Indenture against any officer,
director or member, past, present or future, of the Company as such, either
directly or through the Company, or under any constitution provision, statute or
rule of law or by the enforcement of any assessment or by any legal or equitable
proceedings or otherwise.

        This Series 1999 Lease Note shall not be entitled to any benefit under
the Indenture and shall not become valid or obligatory for any purposes until
the Indenture Trustee shall have signed the form of authentication certificate
endorsed hereon.

        This Series 1999 Lease Note shall expire and be deemed to have been
cancelled upon the earlier of (i) the expiration or early termination of the
Lease with respect to all Units listed on Schedule 1 to the Lease Supplement No.
[___] dated as of [_____], between the Company and the Owner Trustee [identify
Lease Supplement] and (x) if and to the extent required pursuant thereto, the
return of such Units to the Owner Trustee pursuant to the terms and provisions
of the Lease and (y) the making of all payments required to be made by the
Company in connection with the expiration or early termination of the Lease with
respect to the Units, and (ii) payment of all amounts then due and payable on
this Series 1999 Lease Note following the Secured Amount becoming due and
payable hereunder.

        This Series 1999 Lease Note shall be governed by and construed in
accordance with the laws of the State of Georgia.

                                       A-3


<PAGE>


        IN WITNESS WHEREOF, the Company has caused this Series 1999 Lease Note
to be duly executed in its corporate name by its President and Chief executive
Officer and attested by its Secretary and its corporate seal to be hereunto
affixed.

Dated:

                                         OGLETHORPE POWER CORPORATION
                                           (AN ELECTRIC MEMBERSHIP
                                            CORPORATION)

                                         By:
                                            ------------------------------------
                                            Name:
                                            Title:

ATTEST:




----------------------------------
Name:
Title:



                     TRUSTEE'S CERTIFICATE OF AUTHENTICATION

        This is one of the Obligations of the series designated therein referred
to in the within-mentioned Indenture.

                                         SUNTRUST BANK, ATLANTA,
                                         as Trustee

                                         By:
                                           -------------------------------------
                                                Authorized Signatory

                                       A-4

<PAGE>





                                     ANNEX A

                                 SECURED AMOUNT

               Period during which
               Casualty Loss Value
               becomes
               due and payable                     Secured Amount
               ----------------------              --------------

                                       A-5