The geometric and electronic properties of some hydrogenated silicon clusters in the presence of oxygen on the surface have been investigated. The density functional theory with generalized gradient approximation functional was applied in our calculations. By calculating the total energy, the double bond Si=O is shown to be more stable than the bridge bond Si-O-Si for large size oxidized clusters. The results of Mulliken population analysis indicate that a so-called passivation effect is enhanced by oxidization effects. From the energy band structures and density of states, we find that some localized states are induced by the p-orbital of O atom mainly and reduce the energy gaps substantially.