Okla. Stat. tit. 36, § 36-1610: Public structure or improvement obligations

An insurer may invest in bonds, notes, certificates of

indebtedness, warrants, or other evidences of indebtedness, which

are payable from revenues or earnings specifically pledged therefor

of any public structure or improvement owned by any state,

incorporated city, or legally-constituted public corporation or

commission or trust, all within the United States, for the payment

of the principal and interest if no default on the part of the

issuer in payment of principal or interest has occurred on any of

its bonds, notes, warrants, or other securities within five (5)

years prior to the date of investment therein, or, if such

obligations were issued less than five (5) years prior to the date

of investment, no default in payment of principal or interest has

occurred on the obligations to be purchased or on any other

obligation of the issuer within five (5) years of such investment.