Okla. Stat. tit. 19, § 902.15

Bonds and other evidences of indebtedness and the interest thereon shall be paid by revenue derived from an annual levy of not to exceed five (5) mills on the dollar upon the ad valorem taxed property of the district, and all the ad valorem taxed property of the district, including the ad valorem taxed property of public service corporations, shall be and remain liable to be assessed for such payments as herein provided; provided, if Article X, Section 8 of the Oklahoma Constitution requires one hundred percent (100%) of fair cash value to be taxable for purposes of ad valorem taxation, the maximum number of mills authorized by this section shall be three-fourths (3/4) of one (1) mill.