We present a scheme for the daytime ion chemistry of O2 + in the thermosphere and use it to derive moelcular oxygen abundances from ion composition measurements on board the Atmosphere Explorer-C satellite. The molecular oxygen abundance varies from about 1.0¿108 to 5.0¿108 cm-3 at 200 km. No seasonal variation is found, but a significant increase in mean O2 density occurs at high geomagnetic latitudes. We also find a small variation of the O2 density with local time.