Energetic oxygen atoms generated in planar magnetron sputtering was detected by a time-of-flight apparatus which gives information on the energy and the flux of the energetic oxygen atoms. When a Zn target was sputtered in Ar/O-2 gas at 5 mTorr, both the energetic oxygen flux and the degree of oxidation on the Zn target surface increased with increasing oxygen partial pressure. This result indicates that the energetic oxygen atoms and ions are mainly produced by the sputtering of the oxidized ZnO part.