Adsorption energies and structures of a Cu(1 0 0) surface with an oxygen coverage from 0.5 ML to 1.5 ML have been studied using DFT methods. The calculations include both on-surface and mixed on- and sub-surface sites for oxygen. Also several energy barriers for oxygen transfer below the surface have been calculated. Calculations show that mixed structures are energetically more favourable than pure on-surface adsorption at coverages higher than 0.75 ML and barriers for oxygen atoms are low, around 0.5 eV. Experimental results show that during the oxidation process, disordered metal penetrating islands were not seen until after several structural changes, where the oxygen is expected to be on the surface. This is an opposite trend to what we observed in our calculations. An explanation for this discrepancy is that the dissociation of oxygen molecules is very slow on a reconstructed Cu(1 0 0) surface. However, after dissociation, oxygen atoms will diffuse easily below the reconstructed surface and our calculations are in agreement with the experimentally observed disordered islands.