Ab initio calculations of the B3&Sgr;u - potential energy curve of O2 are reported together with the transition moment connecting the B3&Sgr;u - and the ground electronic states. Photodissociation cross sections are presented over the wavelength region 127--152 nm. Small adjustments to the potential energy curve and the transition moment lead to a model of the absorption which agrees well with the experimental cross sections and which reproduces the structural features observed on the high-energy edge. The shoulder at 135.6 nm is attributed to the lowest 3&Pgr;u state and the auxiliary maxima to transitions into the B3&Sgr;u - state.