A detailed picture of dissociation and adsorption of oxygen on the Si(100) reconstructed surface is presented on the basis of the total-energy band structure and force calculations within the local density approximation with use of the normconserving nonlocal pseudopotentials. Dissociation of an oxygen molecule occurs at any site on the Si(100) surface. The resulting oxygen atom is adsorbed on several (meta)stable sites depending on which site the preceding molecule dissociates. Peculiar relaxation of the top-layer Si atoms is found upon oxygen adsorption. Calculated vibrational energy and valence density of states in the most stable geometry are reasonably consistent with the experimental data available, i.e., HREELS and UPS. Finally, we have found that an oxygen molecule penetrates through the oxygen-covered Si(100) surface, on which the Si dangling bonds are terminated, and then dissociates in the vicinity of the Si bond center site.