Okla. Stat. tit. 19 § 1249
The first installment of the assessment, together with interest upon the whole assessment from the date of the passage of the assessing resolution to the first day of the next September, shall be due and payable in cash on or before the first day of September next succeeding the passage of the resolution. If such assessing resolution is not passed prior to the first day of July, the first installment of such assessment shall be due and payable in cash with interest from the date of the passage of such assessing resolution to the first day of September of the following year. In case any installment or interest is not paid when due, the installment so matured and unpaid and the unpaid interest thereon shall draw interest at the rate of fifteen percent (15%) per annum from maturity until paid, except otherwise provided. All assessments and interest shall be collected by the county clerk and shall be paid to the county treasurer who shall keep the same in a separate special fund for the purpose of paying the bonds and interest thereon, issued against such assessments. After the payment of all bonds and interest thereon, any surplus remaining in said fund shall be used for the purpose of repairing and maintaining any improvement for which assessments have been levied, and for no other purpose. No statute of limitations shall commence to run against any installment until after the maturity of all installments.
Okla. Stat. tit. 19, § 1249