Previous determinations of molecular oxygen number densities in the lower thermosphere by ultraviolet extinction in the Schumann-Runge continuum have used absorption cross sections measured at room temperature. Recent laboratory measurements of the temperature dependence of the molecular oxygen absorption cross section in this wavelength region allow a reassessment of the previous atmospheric data. The new cross sections have been used to reanalyze data from a spectrometer flown on an Aerobee rocket from Woomera, Australia. The densities obtained are smaller by as much as 20% than those obtained from the previous analysis, in better agreement with the results of broad band ion chambers flown at the same time that were relatively insensitive to temperature.