An understanding of oxygen adsorption is important for better control of O incorporation and oxidation of nitride semiconductors. Using the density functional method, we demonstrate that under Al-rich conditions Al adatoms or O substitution on N sites can stabilize oxygen adsorption on the (0001) surface. We identify three stabilization mechanisms: the electron counting rule; oxide stoichiometry; and changes in hybridization of the surface Al. In contrast to previous claims, we find that the O adsorption energies exhibit similar trends with increasing coverage for (0001) and surfaces, showing that the energetics of O adsorption do not strongly depend on polarity.