Measurements of molecular oxygen obtained from 1450-¿ absorption data are discussed. Simultaneous observations with independent sensors indicate low random errors in the technique, and systematic erros are also estimated to be low. A survey of spring-fall data at middle and low latitudes during quiet geomagnetic conditions shows an average density variability, including systematic differences in the various techniques, of up to about a factor of 2 from 135 to 180 km. At the higher altitudes the Cira (1972) mean and Jacchia 1971 models tend to overestimate the densities. Data obtained during a moderate geomagnetic disturbance show density enhancements of a factor of 2.5 from 140 to at least 180 km, relative to a typical quiet profile. Although there is more pronounced wave structure in the enhanced profile, no significant changes in the mean scale heights or temperatures are seen. The Jacchia 1971 model does not adequately account for the molecular oxygen density increases during geomagnetic disturbances.