We have performed density functional theory-generalized gradient approximation periodic calculations to study the formation of an oxygen vacancy on rutile-TiO2(110), rutile-SnO2(110), and MgO(100) surfaces assuming that the removed O atom remains adsorbed in the neighborhood of the vacancy (surface Schottky defect). The readsorption of the removed O atom considerably reduces the energetic cost of formation of the vacancy. The readsorption process differs from adsorption of atomic oxygen on a nondefective surface. In fact, on a regular surface an O atom binds to an oxide anion and forms a peroxo group, while on a reduced surface the O atom adsorbs preferentially on the metal cation. The study accounts also for the possible formation of formally charged defects (e.g., the F+ center) at the surface of ionic oxides.