We have investigated the first stages of the room-temperature oxidation of the Si100 surface combining experimental surface optical spectra with the results of ab initio calculations. High-resolution reflectance anisotropy spectra RAS and surface differential reflectance spectra SDRS have been measured for the clean surfaces and various exposures up to 183 L, which have been compared with calculated RAS and SDRS in the independent-particle approximation. Our results, yielding a consistent description of both RAS and SDRS, suggest the coexistence of different structural domains, whose weight changes smoothly with the oxygen exposure. The main oxidation mechanisms together with their occurrence versus coverage are discussed.