OUL is a wide field imager designed as a small, additional payload to be attached to the Luna 26 mission. The instrument has a field of view of 20° × 20° and provides images with angular resolution 3 arcmin in several far ultraviolet bands, including Lyman-α, He II at 164nm and several continuum bands. The imager is designed to monitor the Earth's exosphere and the ecliptic (+/-20°) primary at Lyman-α and in the 125-140 nm and 145-170 nm bands. In this contribution, the optical design of the instrument, its mechanical layout and the science program to be implemented will be described.