The growth of thin Cu films on an O-precovered Ru(0001) surface has been studied by low-energy ion scattering, Auger electron spectroscopy and low-energy electron diffraction. During deposition of Cu, the majority of oxygen (about 70%) originally on the clean Ru(0001) surface was found to float out onto the top of the surface of the growing film. This displacement process could be observed up to a Cu coverage of 10 monolayers, which appeared to be independent of the deposition rate, the O precoverage and the substrate temperature. The floating O atoms in the top layer have been determined to be a disordered overlayer by measuring the azimuthal scan dependence at grazing incidence.