Okla. Stat. tit. 36, § 1615

An insurer may invest in preferred or guaranteed stocks or shares of any solvent institution created or existing under the laws of the United States or of any state, district or territory thereof, if such stock and all of the prior obligations and prior preferred stocks, if any, of such institution at the date of acquisition by such insurer are not then in default. Laws 1957, p. 288, § 1615; Laws 1965, c. 123, § 13.