Okla. Stat. tit. 47 § 12-218.2
Any privately owned motor vehicle operated by a rural letter carrier or any highway contract route vehicle while engaged in the delivery of mail may be equipped with no more than two simultaneously flashing amber lights and a sign reading "U.S. MAIL" for the purpose of warning the operators of other vehicles to exercise care in approaching, overtaking, or passing. Such lights shall be activated as the rural carrier stops on or adjacent to the roadway for the purpose of delivering or collecting United States mail. Such lights shall be of double face or two-way type, be visible when turned on for at least five hundred (500) feet to the front and rear of the vehicle in normal sunlight, be mounted on the highest part of the roof of the vehicle and be spaced laterally as far as practicable to each side of the vehicle. The sign and lights shall be installed so that the sign is lowered and lights turned off before the first stop on the route and following the last one.
Okla. Stat. tit. 47, § 12-218.2