Incoherent scatter data from high solar activity conditions revealed that a large seasonal variation of molecular oxygen at constant pressure was needed to explain the seasonal ionospheric behaviour around 200 km altitude. This suggestion however was not supported by in situ measurements of molecular oxygen abundances onboard Atmosphere Explorer C and D satellites, but during much lower solar activity conditions. It is shown that incoherent scatter data gathered during the same periods of low solar flux are in agreement with the in situ data: the results suggest therefore that the seasonal variation of molecular oxygen is controlled by the solar activity. Part of this control can be linked to the general circulation in the upper atmosphere.