Okla. Stat. tit. 36, § 36-1611: Obligations payable from public utility revenues

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.