The electronic structure and the energetics of oxygen deficient interfaces, constituted by MgO(100) substrates that are capped by epitaxial TiO2 or SrTiO3 layers, are studied within first principles. Various configurations with one O vacancy per (1×1) surface unit cell, which is located either in the MgO or in the SrTiO3 interfacial layers, are compared. The excess electrons either fill Ti states at the bottom of the conduction band or form an F-center as in MgO(100), depending not only upon the side of the interface in which the vacancy is created, but also on the actual vacancy environment. In most cases that we consider, the interface has a metallic character. Our main result regards the order of stability of the various configurations; the oxygen vacancies can be formed more easily in the SrTiO3 overlayer than in the MgO substrate. Moreover, we show that the formation energies of O vacancies can be related to the nature and energy level of the excess electron state. Such a behavior is primarily determined by the interfacial electrostatic potential, which plays a prominent role as in other oxide/oxide interfaces.