Okla. Stat. tit. 70, § 70-15-104: Form - Provision for collection of annual tax

The said bonds shall contain all necessary provisions as to

form; and such school district shall, before or at the time of the

issuance of the same, provide for the collection of an annual tax

sufficient to pay the interest on such indebtedness as it falls due,

and also to constitute a sinking fund for the payment of the

principal thereof.