Thermospheric studies to date have relied heavily on model values of O2 concentration, which are in turn indirectly derived. This paper reports the results of a comparison of a large data base of directly measured O2 concentrations with the concentrations obtained from MSIS model atmosphere. For concentrations of 1010 cm-3 the direct measurements are larger than the model values by up to 50%. For concentrations of 108 cm-3 the measured values are in agreement with the model to within 10%. A comparison is made of the computed major thermospheric species by using both the MSIS and the measured O2 concentrations. The impact is largest in the odd nitrogen species NO, N(4S), and N+, where the difference in concentration can be as much as 70%, and also on O2+.