Observations of the O(3P) fine structure line at 63 ¿m originating in the upper mesosphere and lower thermosphere have been obtained by the far-infrared spectrometer (FIRS-2) instrument, a Fourier transform spectrometer that flies periodically on high-altitude balloons. FIRS-2 primarily observes stratospheric ozone photochemistry using the technique of limb emission spectroscopy. As part of the routine operation of FIRS-2, up-looking views are made, during which the emission from the atomic oxygen is recorded. Using the Mass Spectrometer Incoherent Scatter (MSIS) empirical model to provide temperature and atomic oxygen concentrations, we compute radiances for comparison with the FIRS-2 observations. The computed radiances agree with the FIRS-2 measurements, which encompass 31 observations during nine flights over a span of 14 years, to within 10% on average, with 23 of the 31 observations agreeing to within measurement and calculation uncertainty. The consistency between the observed and computed radiances suggests that the MSIS model provides a reasonably accurate representation of temperature and atomic oxygen in the upper mesosphere and lower thermosphere.