Various kinds of oxygen-adsorbed configurations with oxygen coverages of up to two monolayers (ML) were studied through first-principles calculations to elucidate mechanisms of the layer-by-layer growth and of interfacial defect generation in initial oxidation processes. It was found that oxidation proceeds almost laterally. When the coverage reaches to around 1 ML, first-layer oxidation temporary saturates once, although there still remains oxidation sites in the first-layer. Partial second-layer oxidation opens again a channel of the first-layer oxidation. When the coverage exceeds 1.25 ML, oxygen atoms can be incorporated into a bridging site in the second layer, generating twofold-coordinated Si atoms in the third layer. Emission of such twofold-coordinated Si atoms leaves weakly bonded Si pairs in the fourth layer. When such pairs happen to be generated close to each other, they transform into a chain of Si trimers with one P_b0 center at each end. P_b0 centers appear initially in the fourth layer.