The surface magnetism of an iron film grown on a MgO(1 0 0) substrate has been studied by means of spin polarized metastable de-excitation spectroscopy (SPMDS) as a function of the exposure to oxygen. The O-2p states are magnetically coupled to the iron film and exhibit a splitting in energy DeltaE(O2p) between the majority and the minority components. The behavior of DeltaE(O2p) with oxygen exposure demonstrates that the magnetic coupling of the O-2p states is strongly dependent on the local environment of the adsorbed oxygen atoms and provides a clear indication of the magnetic shielding induced by oxygen diffusion in sub-surface sites.