Okla. Stat. tit. 36, § 1611

An insurer may invest in the bonds, notes, certificates of indebtedness, warrants, or other evidences of indebtedness which are valid obligations issued, assumed, or guaranteed by the United States or any state thereof or by any county, municipal corporation, district, or political subdivision or civil division or public instrumentality of any such government or unit thereof, if by statute or other legal requirements such obligations are payable as to both principal and interest from revenues or earnings from the whole or any part of any utility supplying water, gas, sewage disposal facility or electricity or any other public service. Laws 1957, p. 287, § 1611; Laws 1965, c. 123, § 10.