Photochemically induced departures of [O> and [O2> from diffusive equilibrium are found to depend principally on atmospheric composition, the solar flux between 1250--1750 ¿, and the EUV solar flux which produces O2 dissociation indirectly through odd nitrogen chemistry. Above 200 km the [O2> concentrations are ~20--35% less than diffusive equilibrium concentrations given by a Jacchia model atmosphere, while the O concentrations are enhanced by ~10%.