We present an ab initio study of the effect of iron oxidation on magnetoelectric coupling which appears at the ferromagnet/ferroelectric interface, such as Fe₂/TiO₃(001) (A=Ba, Pb). The oxygen coverage, c, ranging between one half and two adsorbed O per each Fe atom was simulated within a slab geometry. Since the change in magnetization induced by electric polarization reversal remains robust below c<3/2, we suggested that the surface oxidation of composite multiferroics will not significantly damage their switchable magnetoelectricity.