Density functional theory calculations and ambient pressure X-ray photoelectron spectroscopy are used to investigate initial oxidation of Cu(100). Surface stability with respect to oxygen coverage is calculated together with 0 is core level shifts. Oxidation of Cu(100) is found to occur via the formation of a p(2 x 2) overlayer (0.25 ML) followed by a reconstructed (2 root 2 x root 2)R45 degrees-O missing-row (MR) structure (0.50 ML). A c(4 x 6) structure with a 0.3 ML coverage is close in stability for intermediate oxygen chemical potentials. The relative stability is found to be weakly dependent on the applied exchange-correlation functional. The calculated shifts in the O is binding energy are in good agreement with the measured evolution of the binding energy. The shift to higher O is binding energies with increasing oxygen coverage is found to correlate with the charge on neighboring copper atoms. The O is core-level shifts here obtained with CO2 as oxidant, are similar to previous measurements of Cu(100) oxidation with O-2.