Although an immense amount of spectral data regarding the oxygen-adsorbed and oxidized silicon surfaces has accumulated over the past years, many features of the spectra have remained unexplained. In this study, we have investigated the electronic structure of the adsorbed oxygen from the chemisorption stage up to the formation of the oxide layer. Using the empirical tight-binding method we have carried out calculations on the models simulating several binding structures. Comparison made among the calculated electronic structures and the spectra has revealed the following features. (i) In the initial stage, the atomic oxygen chemisorbed on the silicon surface in a head-on position yields state densities which agree with the spectra corresponding to a coverage less than a monolayer. (ii) A binding structure consisting of an atomic oxygen in the bridge position between the first and second layers of the substrate enables us to explain several important features in the observed spectra related with the slow sorption stage. (iii) Adsorbed oxygen induces empty localized states which serve as a final state in the electron-energy-loss spectroscopy. Our results have provided a consistent interpretation of various spectra and an explanation concerning the changes observed in the spectra when one goes from the adsorption stage to the formation of the oxide. Furthermore, we have calculated net charges on the atoms located near the surface and attempted to correlate the shifts of Si 2p core levels in different stages of the adsorption. © 1982 The American Physical Society.