Surface modifications induced by a remote oxygen microwave plasma on three different polymers (LDPE, PS, PMMA) have been extensively investigated in situ by x-ray photoelectron spectroscopy (XPS) and, after transfer in atmosphere, by time of flight secondary ion mass spectrometry (TOF SIMS). By a direct transfer from the plasma chamber to the XPS spectrometer the chemical functionalities introduced on the surface of the polymeric substrate are identified and quantitatively determined. Furthermore, high resolution molecular analysis by TOF SIMS allows a better understanding of the chemical changes induced by the oxygen plasma. The information gathered from the two techniques permits us to propose oxidation mechanisms of the polymer surfaces. The relative density of ground state atomic oxygen O(P3) in the remote discharge is shown to be of fundamental importance for the initiation of the oxidation. This species is responsible for the formation of radicals from which are further produced highly oxidized functionalities (O-C=O, O2C=O). Specific sites of oxidation for the three polymers and a depolymerization mechanism for PMMA are evidenced by TOF SIMS analysis.