<PAGE>

                                                                   Exhibit 10.6a

                                                                  Execution Copy

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

                                  OP Guaranty

                            Dated as of May 1, 2001

                                    made by

                   Resources Capital Management Corporation,
                                 as Guarantor

                             Roseton Units 1 and 2

--------------------------------------------------------------------------------
<PAGE>

                                  OP Guaranty

     This OP GUARANTY, dated as of May 1, 2001 (this "Guaranty"), is made by
                                                      --------
RESOURCES CAPITAL MANAGEMENT CORPORATION, a New Jersey corporation, as guarantor
(the "Guarantor") in favor of the Guaranteed Parties (as defined in Section 4
      ---------
below).

                                  WITNESSETH:

     WHEREAS, the Guarantor is the indirect parent of Roseton OP LLC (the
"Owner Participant");
 -----------------

     WHEREAS, simultaneously herewith the Owner Participant is entering into the
Participation Agreement, dated as of May 1, 2001 (the "Participation
                                                       -------------
Agreement"), among Dynegy Roseton, L.L.C. (the "Company"), Roseton OL LLC (the
---------                                       -------
"Owner Lessor"), the Owner Participant, Wilmington Trust Company, in the
 ------------
capacities referred to therein (the "Lessor Manager"), and The Chase Manhattan
                                     --------------
Bank, as Lease Indenture Trustee and as Pass Through Trustees; and

     WHEREAS, pursuant to the Operative Documents (as described in Section 1
below), this Guaranty is required to be provided by the Guarantor in favor of
the Guaranteed Parties.

     NOW, THEREFORE, in consideration of the foregoing premises and other good
and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the Guarantor agrees for the benefit of each Guaranteed Party, as
follows:

SECTION 1.   DEFINITIONS

     Unless the context hereof shall otherwise require, capitalized terms used
in this Guaranty, including those in the recitals, and not otherwise defined
herein shall have the respective meanings specified in Appendix A to the
Participation Agreement.  The general provisions of Appendix A to the
Participation Agreement shall apply to terms used in this Guaranty.  For the
avoidance of doubt, the term "Operative Documents" as used herein shall mean all
Operative Documents whether in effect on the date of this Guaranty or entered
into after the date of this Guaranty and, in each case, as in effect from time
to time.

SECTION 2.  GUARANTY PROVISIONS

     Section 2.1.   Guaranty. The Guarantor hereby fully, unconditionally and
                    --------
irrevocably guarantees, as primary obligor and not merely as a surety, on a
senior unsecured basis (pari passu with all other senior unsecured indebtedness
of the Guarantor), (a) the due and punctual performance and observance by the
Owner Participant of each term, provision and condition binding upon or
applicable to the Owner Participant under or pursuant to any of the Operative
Documents (including, without limitation, interest at the then applicable rate
provided in the Lease Indenture after the filing of any petition in bankruptcy,
or the commencement of any insolvency, reorganization or like proceeding,
relating to the Owner Participant, whether or not a
<PAGE>

claim for post-filing or post-petition interest is allowed in such proceeding)
(the "Performance Obligations"), and (b) the due, punctual and full payment
(when and as the same may become due and payable) of each amount that the Owner
Participant is or may become obligated to pay under or pursuant to any of the
Operative Documents, in accordance with the terms thereof (the "Payment
Obligations"), by acceleration or otherwise without offset or deduction. In the
case of any failure by the Owner Participant to perform or observe the
Performance Obligations after notice thereof by any Guaranteed Party, the
Guarantor agrees to cause such performance or observance to be done, and in the
case of any failure by the Owner Participant to make Payment Obligations as and
when the same shall become due and payable (by acceleration or otherwise), the
Guarantor hereby agrees to make such payment (and, in addition, such further
amounts, if any, as shall be sufficient to cover any and all costs and expenses,
including reasonable legal fees, of collection and enforcement hereunder);
provided, that nothing herein shall expand the aforesaid obligations of the
Guarantor beyond those of the Owner Participant under the Operative Documents.

     All Performance Obligations and the Payment Obligations are collectively
referred to in this Guaranty as the "Obligations."
                                     -----------

     The Guarantor hereby acknowledges and agrees that this Guaranty constitutes
a continuing guaranty and shall remain in full force and effect until such time
as all of the Obligations are finally paid, performed and observed in full.  The
Guarantor hereby further acknowledges and agrees that this Guaranty constitutes
a guaranty of payment and performance when due and not of collection and waives
any right to require that any resort be had by any Guaranteed Party against any
other obligor, to any of the security held for payment of the Obligations or to
any balance of any deposit account or credit on the books of any Guaranteed
Party in favor of the Owner Participant or any other person or against any
guarantor under any other guarantee covering the Obligations.

     Section 2.2.   Guaranty Absolute, etc.  The obligations of the Guarantor
                    ----------------------
contained herein are direct, independent and primary obligations of the
Guarantor and are absolute, present, full, unconditional and continuing
obligations and are not conditioned in any way upon the institution of suit or
the taking of any other action or any attempt to enforce performance of or
compliance with the Obligations of the Owner Participant other than providing
notice to the Owner Participant to the extent provided in Section 2.1 with
respect to Performance Obligations and shall constitute a guaranty of payment
and performance and not only of collection, binding upon the Guarantor and its
successors and assigns and shall remain in full force and effect and irrevocable
without regard to the genuineness, validity, legality or enforceability of any
of the Operative Documents to which the Owner Participant is a party or any lack
of power or authority of the Owner Participant to enter into any of the
Operative Documents to which the Owner Participant is a party or, except as
expressly provided in Section 7.1(h)(ii) of the Participation Agreement, any
substitution, release or exchange of any other guaranty or any other security
for any of the Obligations or any other circumstance whatsoever (other than full
payment or performance) that might otherwise constitute a legal or equitable
discharge or defense of a surety or guarantor and shall not be subject to any
right of set-off, recoupment or counterclaim and are in no way conditioned or
contingent upon any attempt to collect from the Owner Participant or any other
entity or to perfect or enforce any security or upon any other condition or
contingency or upon any other action, occurrence or circumstance whatsoever.
Without limiting the

                                       2
<PAGE>

generality of the foregoing, and subject to Section 7.1(h)(ii) of the
Participation Agreement, the Guarantor shall have no right to terminate this
Guaranty, or to be released, relieved or discharged from its obligations
hereunder, and such obligations shall be neither affected nor diminished for any
reason whatsoever (other than full payment or performance of the Obligations),
including (i) any amendment or supplement to or modification of any of the
Operative Documents, any extension or renewal of the Owner Participant's
obligations under any Operative Document, or subject to Section 7.1(h)(ii) of
the Participation Agreement, any subletting, assignment or transfer of the Owner
Participant's or any Guaranteed Party's interest in the Operative Documents,
(ii) any bankruptcy, insolvency, readjustment, composition, liquidation or
similar proceeding with respect to the Owner Participant or any other Person,
(iii) any furnishing or acceptance of additional security or any exchange,
substitution, surrender or release of any security, (iv) any waiver, consent or
other action or inaction or any exercise or nonexercise of any right, remedy or
power with respect to the Obligations or any of the Operative Documents, (v) any
merger or consolidation of the Owner Participant or the Guarantor into or with
any other Person, or any change in the structure of the Owner Participant or in
the ownership of the Owner Participant by the Guarantor, or any sale, lease or
transfer of any or all of the assets of the Owner Participant or the Guarantor
to any other Person, or (vi) any default, misrepresentation, negligence,
misconduct or other action or inaction of any kind by any Guaranteed Party under
or in connection with any Operative Document or any other agreement relating to
this Guaranty, except to the extent that any such default, misrepresentation,
negligence, misconduct or other action or inaction would limit the Obligations.
The Guarantor hereby unconditionally waives to the extent permitted by law
promptness, diligence and notice as to the Obligations guaranteed hereby and
acceptance of this Guaranty, and agrees that, except as otherwise provided
herein, it shall not be required to consent to or receive any notice of any
amendment or modification of, or waiver, consent or extension with respect to,
the Operative Documents. The rights, powers and remedies herein provided are
cumulative. No failure or delay on the part of any Guaranteed Party in
exercising any right, power or privilege hereunder shall operate as a waiver
thereof, nor shall any single or partial exercise of any right, power or
privilege hereunder preclude any other or further exercise of any other right,
power or privilege. The Guarantor agrees to pay any costs and expenses
reasonably incurred by the parties to the Participation Agreement or any
Guaranteed Party in connection with the enforcement of this Guaranty.

     Section 2.3.   Operative Documents.  The Guarantor does hereby acknowledge
                    -------------------
that it is aware of the terms and conditions of the Operative Documents and the
transactions and the other documents contemplated thereby.

     Section 2.4.   Payment.  All payments to be made by the Guarantor hereunder
                    -------
shall be made in immediately available funds and in dollars to the Guaranteed
Parties to which such payment is to be made and shall, as to any Guaranteed
Party that is a United States person (within the meaning of Section 7701(a)(30)
of the Code) at the time of payment, be free and clear of any deduction or
withholding.  All such payments shall be paid to such Guaranteed Party in the
manner and at the place required by the Operative Documents or, if no such
address is provided, at the address and to the accounts specified in the notice
demanding payment be made by Guarantor.

                                       3
<PAGE>

SECTION 3.  REPRESENTATIONS AND WARRANTIES

     The Guarantor represents and warrants that as of the date of this Guaranty
and as of the Closing Date:

     Section 3.1.   Due Organization, Etc.  The Guarantor is a corporation duly
                    ---------------------
organized, validly existing, and in good standing under the laws of the State of
New Jersey, is duly qualified to transact business and in good standing in each
jurisdiction in which the failure so to qualify would have a material adverse
effect, and has the corporate power and authority to enter into and perform its
obligations under this Guaranty.

     Section 3.2.   Due Authorization, Enforceability, Etc.  This Guaranty has
                    --------------------------------------
been duly authorized, executed and delivered by all necessary corporate action
by the Guarantor and constitutes the legal, valid and binding obligation of the
Guarantor, enforceable against the Guarantor in accordance with the terms
hereof, except as the same may be limited by bankruptcy, insolvency, fraudulent
conveyance, reorganization, arrangement, moratorium or other laws relating to or
affecting the rights of creditors generally and by general principles of equity.

     Section 3.3.   Non-Contravention.  The execution and delivery by the
                    -----------------
Guarantor of this Guaranty, and the consummation by the Guarantor of the
transactions contemplated hereby, and the compliance by the Guarantor with the
terms and provisions hereof, do not and will not (i) contravene (A) any
Applicable Law binding on the Guarantor or its property, or (B) its
organizational documents, or (ii) constitute a default by the Guarantor under,
or result in the creation of any Lien upon the property of the Guarantor (other
than as permitted pursuant to any Operative Document) under, any indenture,
mortgage or other material contract, agreement or instrument to which the
Guarantor is a party or by which the Guarantor or any of its property is bound,
which in the case of clause (i)(A) or (ii) of this Section 3.3, individually or
in the aggregate, is reasonably likely to result in a material adverse effect.

     Section 3.4.   Litigation.  There is no pending or, to the Actual Knowledge
                    ----------
of the Guarantor, threatened, action, suit, investigation or proceeding against
the Guarantor before any Governmental Entity which questions the validity of
this Guaranty or the ability of the Guarantor to perform its obligations
hereunder.

     Section 3.5.   No Other Consent Required.  No consent from any Person is
                    --------------------------
required for the execution, delivery and performance by the Guarantor of this
Guaranty except that which has been given.

SECTION 4.  GUARANTEED PARTIES

     Each of the Company, the Owner Lessor, the Lessor Manager, the Trust
Company and, so long as the Lien of the Lease Indenture has not been terminated
or discharged, the Lease Indenture Trustee, in each case, together with any
Indemnitee related thereto and their respective successors and permitted
assigns, are each Guaranteed Parties of this Guaranty (each, a "Guaranteed
                                                                ----------
Party" or, together, the "Guaranteed Parties").
                          ------------------

                                      4
<PAGE>

SECTION 5.  SURVIVAL OF GUARANTY

     Notwithstanding anything to the contrary herein, this Guaranty shall
continue to be effective or be reinstated, as the case may be, if at any time
any of the amounts paid to any of the Guaranteed Parties, in whole or in part,
is required to be repaid upon the insolvency, bankruptcy, dissolution,
liquidation, or reorganization of the Guarantor or the Owner Participant or any
other Person, or as a result of the appointment of a custodian, interviewer,
receiver, trustee, or other officer with similar powers with respect to the
Guarantor or the Owner Participant or any other Person or any substantial part
of the property of the Guarantor or the Owner Participant or such other Person,
all as if such payments had not been made.

SECTION 6.  REMEDIES; SUBROGATION

     Section 6.1.   Remedies.  In the event the Guarantor shall fail to pay
immediately any amounts due under this Guaranty, or to comply with any other
term of this Guaranty, each Guaranteed Party shall be entitled to all rights and
remedies to which it may be entitled hereunder or at law, in equity or by
statute.

     Section 6.2.   Subrogation. The Guarantor will not exercise any rights that
it may acquire by way of subrogation under this Guaranty, by any payment made
hereunder or otherwise, until all of the Obligations shall have been
indefeasibly paid in full. If any amount shall be paid to the Guarantor on
account of such subrogation rights at any time when all of the Obligations shall
not have been indefeasibly paid in full, such amount shall be held in trust for
the benefit of the Guaranteed Party to whom such Obligation is payable and shall
forthwith be paid to such Guaranteed Party to be credited and applied to such
Obligation, in accordance with the terms of the Operative Document under which
such Obligation arose, when such Obligation is due and payable. If (i) the
Guarantor shall make payment to any Guaranteed Party of all or any part of the
Obligations and (ii) all the Obligations shall be paid in full, such Guaranteed
Party will, at the Guarantor's request and expense, execute and deliver to the
Guarantor appropriate documents, without recourse and without representation or
warranty, necessary to evidence the transfer by subrogation to the Guarantor of
an interest in the Obligations resulting from such payment by the Guarantor.

     Section 6.3.   Survival of Remedies and Subrogation Rights. The provisions
of this Section 6 shall survive the termination of this Guaranty and the payment
in full of the Obligations and the termination of the Operative Documents.

SECTION 7.  MISCELLANEOUS

     Section 7.1.   Amendments and Waivers.  No term, covenant, agreement or
condition of this Guaranty may be terminated, amended or compliance therewith
waived (either generally or in a particular instance, retroactively or
prospectively) except by an instrument or instruments in writing executed by the
Guarantor and consented to by the Company, the Owner Lessor, the Lessor Manager,
the Trust Company and, so long as the Lien of the Lease Indenture has not been
terminated or discharged, the Lease Indenture Trustee.

     Section 7.2.   Notices. Unless otherwise expressly specified or permitted
by the terms hereof, all communications and notices provided for herein shall be
in writing or by a

                                       5
<PAGE>

telecommunications device capable of creating a written record, and any such
notice shall become effective (a) upon personal delivery thereof, including by
overnight mail or courier service, (b) in the case of notice by United States
mail, certified or registered, postage prepaid, return receipt requested, upon
receipt thereof, or (c) in the case of notice by such a telecommunications
device, upon transmission thereof, provided such transmission is promptly
confirmed by either of the methods set forth in clauses (a) or (b) above, in
each case addressed to the Guarantor hereto at its address set forth below or at
such other address as such party may from time to time designate by written
notice.

If to the Guarantor:

     Resources Capital Management Corporation
     1300 North Market Street, Suite 405
     Wilmington, DE  19801
     Telephone: (302) 576-2895
     Facsimile: (302) 576-2897
     Attention: William R. Barbour, Esq.

If to the Guaranteed Parties:

     To the address provided for such party
     from time to time in or pursuant to
     the Participation Agreement.

     Section 7.3.   Survival.  Except as expressly set forth herein, the
warranties and covenants made by the Guarantor shall not survive the termination
of this Guaranty.

     Section 7.4.   Assignment and Assumption. This Guaranty may not be assigned
by the Guarantor to, or assumed by, any successor to or assign of the Guarantor
without the prior written consent of the Guaranteed Parties except (i) in
connection with a merger or consolidation of the Guarantor into another Person,
or the sale of all or substantially all of the Guarantor's assets to a Person,
pursuant to an assignment and assumption agreement in form and substance
reasonably satisfactory to the Company, the Owner Lessor, the Lessor Manager,
the Trust Company and, so long as the Lien of the Lease Indenture has not been
terminated or discharged, the Lease Indenture Trustee, of the Guarantor's
obligations hereunder (or a new guaranty substantially in the form of Exhibit H-
1 to the Participation Agreement) or (ii) as otherwise provided by the terms of
the Participation Agreement. Notwithstanding the foregoing, prior to the
termination of the Lien of the Lease Indenture the Guarantor covenants and
agrees that the Guarantor will not transfer its direct or indirect interest in
the Owner Lessor (including by way of the merger of the Guarantor), unless after
giving effect to the transfer, the entity that is the beneficial owner (for
United States federal income tax purposes) of the amounts payable to the Owner
Lessor under the Facility Lease shall be a "United States person" within the
meaning of Section 7701(a)(30) of the Code. In addition, the Guarantor shall
neither make nor permit to be made nor fail to make nor permit the failure to
make any election that would cause the entity that is the beneficial owner (for
United States federal income tax purposes) of the amounts payable to the Owner
Lessor under the Facility Lease not to be a "United States person" within the
meaning of Section 7701(a)(30) of the Code.

                                       6
<PAGE>

     Section 7.5.   Governing Law. This Guaranty shall be in all respects
governed by and construed in accordance with the laws of the State of New York,
including all matters of construction, validity and performance (without giving
effect to the conflicts of laws provisions, other than New York General
Obligations Law Section 5-1401).

     Section 7.6.   Consent to Jurisdiction; Waiver of Trial by Jury.  (a)  The
Guarantor (i) hereby irrevocably submits to the nonexclusive jurisdiction of the
Supreme Court of the State of New York, New York County (without prejudice to
the right of any party to remove to the United States District Court for the
Southern District of New York) and to the nonexclusive jurisdiction of the
United States District Court for the Southern District of New York for the
purposes of any suit, action or other proceeding arising out of this Guaranty,
or the subject matter hereof or any of the transactions contemplated hereby
brought by any of the Guaranteed Parties or their successors or assigns; (ii)
hereby irrevocably agrees that all claims in respect of such action or
proceeding may be heard and determined in such New York State court, or in such
federal court; and (iii) to the extent permitted by Applicable Law, hereby
irrevocably waives, and agrees not to assert, by way of motion, as a defense, or
otherwise, in any such suit, action or proceeding any claim that it is not
personally subject to the jurisdiction of the above-named courts, that the suit,
action or proceeding is brought in an inconvenient forum, that the venue of the
suit, action or proceeding is improper or that this Guaranty, or the subject
matter hereof may not be enforced in or by such court.

              (b)   TO THE EXTENT PERMITTED BY APPLICABLE LAW, THE GUARANTOR
HEREBY IRREVOCABLY WAIVES THE RIGHT TO DEMAND A TRIAL BY JURY, IN ANY SUCH SUIT,
ACTION OR OTHER PROCEEDING ARISING OUT OF THIS GUARANTY, OR THE SUBJECT MATTER
HEREOF OR ANY OF THE TRANSACTIONS CONTEMPLATED HEREBY BROUGHT BY ANY OF THE
GUARANTEED PARTIES OR THEIR SUCCESSORS OR ASSIGNS.

     Section 7.7.   Severability. Any provision of this Guaranty that is
prohibited or unenforceable in any jurisdiction shall, as to such jurisdiction,
be ineffective to the extent of such prohibition or unenforceability without
invalidating the remaining provisions hereof, and any such prohibition or
unenforceability in any jurisdiction shall not invalidate or render
unenforceable such provision in any other jurisdiction.

     Section 7.8.   Headings.  The headings of the sections of this Guaranty are
inserted for purposes of convenience only and shall not be construed to affect
the meaning or construction of any of the provisions hereof.

     Section 7.9.   Further Assurances.  The Guarantor will promptly and duly
execute and deliver such further documents to make such further assurances for
and take such further action reasonably requested by any of the Guaranteed
Parties referred to in Section 4 to whom the Guarantor is obligated, all as may
be reasonably necessary to carry out more effectively the intent and purpose of
this Guaranty.

     Section 7.10.  Effectiveness of Guaranty.  This Guaranty has been dated as
of the date first above written for convenience only.  This Guaranty shall be
effective on May 1, 2001, the date of execution and delivery by the Guarantor.

                                       7
<PAGE>

     Section 7.11.   Successors and Assigns. This Guaranty shall be binding upon
the Guarantor and its successors and permitted assigns and shall inure to the
benefit of, and shall be enforceable by, each of the Guaranteed Parties and
their respective successors and permitted assigns.

                                       8
<PAGE>

          IN WITNESS WHEREOF, the Guarantor has caused this Guaranty to be duly
executed and delivered by its officer thereunto duly authorized.

                               RESOURCES CAPITAL
                               MANAGEMENT CORPORATION,
                               as Guarantor

                               By:________________________________
                                  Name:
                                  Title: