We present a theoretical density-functional theory study of the effect of deposition on a metal substrate (Mg, Pt, Ag, and Au) on the phase diagram of ultrathin MgO(111) polar films. By considering various crystallographic structures (rocksalt, zinc-blende, and graphiticlike), layer stackings and lattice registries at the interface, we identify the two principal mechanisms by which oxide films interact with the metal substrate. On one hand, metal provides the charges necessary for polarity compensation at the interface, enables the oxide ions at the interface to recover their usual valence, and induces a strong stabilization of polar films. On the other hand, due to interfacial bonding and band alignment (function of the metal electronegativity), a second type of electron exchange occurs at the interface, dominant in films which are either nonpolar or uncompensated polar. It induces a structural distortion of the oxide film in the interface region and yields a nonvanishing film polarization. The stabilizing effect of the metal substrate is considerably weaker in this case.