Okla. Stat. tit. 45, § 939
All entities providing electric power for sale to the consumer in Oklahoma and generating said power from coal-fired plants located in Oklahoma shall burn a mixture of coal that contains a minimum of ten percent (10%) Oklahoma mined coal, as calculated on a BTU (British Thermal Unit) basis.
Laws 1986, SB 458, c. 43, § 1, eff. January 1, 1987.