We derive molecular oxygen densities in the atmosphere from 140 to 240 km using measurements of total oxygen atom concentrations made on board the Atmosphere Explorer C satellite by the open source neutral mass spectrometer. A Bates-Walker profile of atomic oxygen densities was obtained by using a fit to the densities measured at high altitudes. At lower altitudes the difference between the measured total oxygen atom concentration and the value from the diffusive profile is approximately twice the density of molecular oxygen. The predicted O2 densities are generally in harmony with values of the O2 density derived from ion composition measurments.