Okla. Stat. tit. 62, § 391

Every county, every city or town, the board of education of every city, every township, and every school district, is hereby authorized and empowered to refund its indebtedness, including bonds, judgments and warrants, as hereinafter provided, upon such terms as can be agreed upon, and to issue new bonds with interest in payment for any sum so refunded; which bonds shall be sold at no less than par, and shall not be for a longer period then twenty-five (25) years, shall not exceed in amount the actual amount of outstanding indebtedness, inclusive of accrued interest and shall not draw a greater interest than the maximum rate provided by Section 498.1 of this title.