In any case where refunding bonds are issued and sold six (6) months or more before the earliest date on which all bonds to be refunded thereby mature or are called for redemption in accordance with their terms, the proceeds of the refunding bonds, other than the amount included therein incidental to the issuance of the bonds, shall be deposited, together with any other funds available and appropriated by the board for the purpose, in escrow with a suitable banking institution having trust powers within the state, whose deposits are insured by the Federal Deposit Insurance Corporation. Such monies shall be invested in securities maturing or callable at the option of the holder on such dates and bearing interest at such rates as shall be required to provide funds sufficient, with any cash retained in the escrow account, to pay when due the interest to accrue on each obligation refunded to its maturity, or if prepayable, to an earlier designated date on which it may be called for redemption, and to pay the principal amount of each such bond at maturity, or, if prepayable at its designated earlier redemption date, and to pay any premium required for redemption on such date. Before the refunding bonds are delivered, the board shall by resolution irrevocably appropriate for these purposes the escrow account and all payments of principal and interest on the securities held therein, and shall provide for the call of all bonds directed to be prepaid, in accordance with their terms, on the redemption date or dates designated. Securities purchased from the escrow account shall be limited to direct obligations of the United States or obligations whose principal and interest payments are guaranteed by the United States. Such securities shall be purchased simultaneously with the delivery of the refunding bonds. No refunding bonds shall be issued more than ten (10) years before the last date on which the bonds to be refunded thereby mature or are directed to be prepaid in accordance with their terms. All bonds issued under this paragraph and the preceding paragraph shall in all respects be authorized, issued and secured in the manner provided for other bonds issued under this article, and shall have all of the attributes of such bonds. The board may provide that any such refunding bonds shall have the same priority of lien on the revenues pledged for their payment as was enjoyed by the bonds refunded thereby.
Okla. Stat. tit. 70, § 4002