Okla. Stat. tit. 36 § 1610
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.
Okla. Stat. tit. 36, § 1610