Relying on an ab initio approach to model MgO/metal interfaces, we unravel the specificity of polarization effects in the oxide monolayer limit. We show that rumpling of the supported oxide film is a structural response to the interfacial charge transfer. Surface characteristics of such complex supports can thus be directly tuned by an adequate choice of the metal and the oxide. This mechanism equally affects films of nonpolar and polar orientations. Contrary to thicker films, there is thus no electrostatic signature distinguishing the two types of orientation.