In the present paper we discuss the defects at the oxide/Si interface and the structure of silicon oxide films grown on plasma hydrogenated (100) and (111)Si. The effect of oxide thickness ranging from 7 to 40 nm on the interface parameters was examined. Electrically active defects were characterized through C-V and G-V measurements. The dependence of the refractive index on oxide thickness was studied. Information on the oxide structure was inferred through the refractive index evaluated from ellipsometric measurements. From both, the electrical and optical results a characteristic oxide thickness was found, below which the oxide structure is different from SiO2, most probably SiOх. It is related to a modified Si surface during the pre-oxidation plasma treatment and its value depends on Si orientation and pre-clean conditions. A characteristic oxide thickness of 13 nm was found for Si hydrogenated without heating and, of 9 nm for Si hydrogenated at 300oC.