Ni films between 1 and 20 monolayers (ML) thick are deposited at room temperature on clean and (√2x2√2)R45 o reconstructed--via oxygen adsorption--Cu(001). A significant expansion of the out-of-plane Ni phase by about 5 ML is revealed by ferromagnetic resonance experiments. This shift of the spin reorientation transition is attributed to a huge change of about 90 μeV/atom in the surface anisotropy due to the presence of half a monolayer of oxygen atoms on the top of Ni. Furthermore, the growth of Ni on the preoxidized Cu surface is found to be closer to the layer-by-layer one as compared to the growth on the clean Cu(001) due to the presence of oxygen which acts as a surfactant.