Atomic oxygen densities, [O], have been retrieved from Optical Spectrograph and Infrared Imaging System (OSIRIS) observations of the O2 (b1Σg+, v′ = 0 − X3Σg+, v″ = 0) A-band airglow emission in the mesosphere and lower thermosphere. Daytime atomic oxygen densities, [O], have been inferred from ozone densities, [O3], that were retrieved from OSIRIS O2 A-band dayglow observations, and nighttime [O] have been retrieved directly from A-band nightglow observations employing an iterative-Newtonian optimal estimation inversion technique. Low-latitude results from 2007–2009 are presented and discussed. Variations in [O] between daytime (early morning, ∼0700 LT) and nighttime (late evening, ∼1900 LT) are shown to be strongly altitude dependent, with average nighttime densities in the tropics larger than daytime densities by ∼10% at 95 km and by more than a factor of 4 at 85 km. Seasonal variations in [O] are also examined and indicate that the seasonal variations of [O] are strongly influenced by the local photochemistry and are less dependent on dynamical variations. OSIRIS measurements are also compared to [O] predicted by the NRL-MSISE-00 model. The comparisons show that the derived OSIRIS densities are up to an order of magnitude larger than the MSIS values and that the seasonal oscillations of MSIS [O] are 180° out of phase with the OSIRIS measurements.