Understanding the physics of unintentional doping and defect formation during epitaxial growth of III‐nitride semiconductors is crucial to develop optical and electronic devices. Herein, the impact of magnesium doping on unintentional oxygen incorporation into GaN and AlN during metal‐organic vapor‐phase epitaxy is investigated by first‐principles calculations. It is found that the presence of Mg substituting group‐III atoms (Ga or Al) in subsurface layers energetically promotes unintentional oxygen incorporation. The calculation results also suggest that even when Mg + H complex defects exist in subsurface layers, they promote unintentional oxygen incorporation in a similar manner. The mechanism of unintentional oxygen incorporation enhanced by magnesium doping and complex defect structures is discussed in terms of charge neutrality or electron‐counting model in the growth surface.