Summer hot oxygen despities of 1 to 3¿104 cm-3 at 1100 km (or 1 to 3¿105 at 550 km) for low to moderate solar activities are inferred from differences between satellite drag and mass spectrometer based models. At high solar activities, densities at 925 km from 1 to 4¿105 (or 5¿105 to 2¿106 at 550 km) are inferred from a limited number of Dynamics Explorer mass spectrometer measurements. The densities are the same order as those reported previously from ground based optical measurements. Alternative explanations for model differences, such as errors in mass spectrometer data or in the temperature profile, are unlikely to satisfactorily explain model differences in the high-altitude summer hemisphere without severe discrepancies in other areas. A hot oxygen population in the summer hemisphere with a scale height similar to helium would be interpreted in analysis of drag data as if it were helium and explain the underestimation of winter to summer helium variation in drag based models. Hot oxygen could be providing the major contribution to satellite drag in the summer lower exosphere during low solar activity. ¿ American Geophysical Union 1989