Okla. Stat. tit. 82, § 688

All properties owned by the district, both real and personal, and reasonably necessary to accomplish the purposes of district shall be exempt from taxation by the State of Oklahoma, or by any municipal corporation, county or other political subdivision. All properties, products and benefits sold, leased or furnished by such districts shall be exempt from sales tax. All bonds, notes and warrants and the interest thereon, issued pursuant to the provisions of this act, shall be exempt from taxation (except inheritance taxes) by the State of Oklahoma, or by any municipal corporation, county or other political subdivision, or taxing district of the state.