Adsorption of oxygen on a polycrystalline titanium surface at room temperature has been studied by metastable deexcitation spectroscopy in conjunction with ultraviolet photoelectron spectroscopy and Auger electron spectroscopy. From the analysis of the obtained spectra, we have shown the following. (1) With increasing oxygen exposure at <2 L, the surface density of states decreases substantially 1-2 eV below the Fermi level(EF) but it undergoes a small change at around the EF. (2) The TiO2 phase appears on the outermost titanium surface at around 5 L. It grows with increasing oxygen exposure, but it is a minor phase even at >100 L.