Okla. Stat. tit. 36, § 1609

An insurer may invest in bonds, notes, warrants and other securities not in default of any county, district, incorporated city, or school district in any state of the United States, or the District of Columbia, or in any province of Canada, which are the direct obligations of such county, district, city or school district and for payment of the principal and interest of which the county, district, city, or school district has lawful authority to levy taxes or make assessments. Bonds, notes, warrants and other securities classified as revenue, prerefunded or declining balances are not considered acceptable investments for this purpose.