Okla. Stat. tit. 36, § 6401

As a condition of doing business in the State of Oklahoma, any insurer duly licensed in Oklahoma and who, on the effective date of this act, is writing custom harvesting insurance in any state in the United States or who has written such insurance in the State of Oklahoma within the past three (3) years shall provide such insurance to those custom harvesters who are unable to procure such insurance in the open or excess insurance market.