Neutral atomic oxygen densities [O> in the thermosphere are calculated from analysis of coincident Millstone Hill incoherent scatter radar and Fabry Perot interferometer measurements taken during 14 nights with widely varying geomagnetic conditions in 1990 and 1991, and the results are compared with the mass spectrometer/incoherent scatter 1986 (MSIS-86) model. The results are generally satisfactory except during the most rapidly varying conditions, when some of our assumptions break down, such as those of a uniform ion velocity field and no vertical neutral wind. On the average, the results agree well with MSIS-86. However, systematic differences are found on two of the geomagnetically disturbed nights, with the observed [O> much smaller than the MSIS-86 values on one night, and much larger on another. ¿American Geophysical Union 1992