The open source neutral mass spectrometers on the AE-C, -D, and -E satellites were equipped with a 'fly-through' mode of operation which has provided direct measurements of molecular oxygen densities over a large portion of the globe. A complementary set of O2 densities is derived by using AE ion measurements and a scheme based on the daytime ion chemistry of O2+ in the thermosphere. A comparison of the two data sets reveals general agreement over northern latitudes during periods of relatively low Ap and F10.7. The simplifying assumptions made in the photochemcial scheme require that caution be used in calculating [O2>, especially at high latitudes and altitudes below 200 km.