Stable structures of polar, nonpolar, and semipolar GaN surfaces undergoing oxide vapor phase epitaxy (OVPE) growth were examined with firstprinciples calculations. The relationships between temperature and pressure growth conditions and stable surface structures are described in surface phase diagrams. The results revealed that an O atom was stably incorporated into the N vacancy site in the N-polar and nonpolar surfaces. The desorption energy of the O atom from the GaN surfaces was estimated to be about 7 eV or higher. This indicates that the O atom did not readily desorb from the GaN surfaces under OVPE growth conditions at 1500 K. The desorption energy from the  surface was the highest and that from the  surface was the smallest among the calculated values. There was no significant difference in desorption energy among other surfaces.