Exhibit 5



                                                                 April 20, 2004


Public Service Company of Oklahoma
1 Riverside Plaza
Columbus, Ohio  43215

Ladies and Gentlemen:

                  We have acted as counsel to Public Service Company of
Oklahoma, an Oklahoma corporation (the "Company"), in connection with the
Registration Statement on Form S-3 (the "Registration Statement") filed by the
Company with the Securities and Exchange Commission (the "Commission") under the
Securities Act of 1933, as amended (the "Act"), relating to Unsecured Notes (the
"Unsecured Notes") to be issued under an Indenture, dated as of November 1, 2000
(the "Indenture"), between the Company and The Bank of New York, as Trustee (the
"Trustee"). The Unsecured Notes may be issued and sold or delivered from time to
time as set forth in the Registration Statement, any amendment thereto, the
prospectus contained therein (the "Prospectus") and supplements to the
Prospectus and pursuant to Rule 415 under the Act for an aggregate initial
offering price not to exceed $200,000,000.

                  We have examined the Registration Statement and the Indenture
which has been filed with the Commission as an exhibit to the Registration
Statement. We also have examined the originals, or duplicates or certified or
conformed copies, of such corporate records, agreements, documents and other
instruments and have made such other investigations as we have deemed relevant
and necessary in connection with the opinions hereinafter set forth. As to
questions of fact material to this opinion, we have relied upon certificates or
comparable documents of public officials and of officers and representatives of
the Company.

                  In rendering the opinions set forth below, we have assumed the
genuineness of all signatures, the legal capacity of natural persons, the
authenticity of all documents submitted to us as originals, the conformity to
original documents of all documents submitted to us as duplicates or certified
or conformed copies and the authenticity of the originals of such latter
documents. We also have assumed that: (1) the Indenture is the valid and legally
binding obligation of the Trustee; and (2) the Company is validly existing under
the laws of Oklahoma.

                  We have assumed further that (1) the Company has duly
authorized, executed and delivered the Indenture and (2) execution, delivery and
performance by the Company of Indenture and the Unsecured Notes do not and will
not violate the laws of Oklahoma or any other applicable laws (excepting the
laws of the State of New York and the Federal laws of the United States).

                  Based upon the foregoing, and subject to the qualifications
and limitations stated herein, we are of the opinion that: assuming (a) the
taking of all necessary corporate action to approve the issuance and terms of
the Unsecured Notes, the terms of the offering thereof and related matters by
the Board of Directors of the Company, a duly constituted and acting committee
of such Board or duly authorized officers of the Company (such Board of
Directors, committee or authorized officers being referred to herein as the
"Board") and (b) the due execution, authentication, issuance and delivery of
such Unsecured Notes, upon payment of the consideration therefore provided for
in the applicable definitive purchase, underwriting or similar agreement
approved by the Board and otherwise in accordance with the provisions of the
Indenture and such agreement, such Unsecured Notes will constitute valid and
legally binding obligations of the Company enforceable against the Company in
accordance with their terms, subject to the effects of (i) bankruptcy,
insolvency, fraudulent conveyance, reorganization, moratorium and other similar
laws relating to or affecting creditors' rights generally, (ii) general
equitable principles (whether considered in a proceeding in equity or at law)
and (iii) an implied covenant of good faith and fair dealing.

                  We do not express any opinion herein concerning any law other
than the law of the State of New York and the Federal law of the United States.

                  We hereby consent to the filing of this opinion letter as
Exhibit 5 to the Registration Statement and to the use of our name under the
caption "Legal Opinions" in the Prospectus included in the Registration
Statement.

                                            Very truly yours,

                                            /s/ Simpson Thacher & Bartlett LLP
                                            SIMPSON THACHER & BARTLETT LLP