The capturing properties of nonbridging oxygen hole centers with unpaired electrons ≡SiO· and hydrogen defects ≡SiOH in silicon oxide have been studied using the ab initio density-functional method. It was found that the ≡SiO· defect is an electron trap and should be the responsible candidate for better hardness against radiation for the metal-oxide-semiconductor gate oxide produced by wet oxidation. On the other hand, the ≡SiOH defect, which was proposed to be an electron or "water trap" in the oxide (A. Hartstein and D. R. Young, Appl. Phys. Lett., vol. 38, pp. 631, 1981), could not be an electron trap according to the present calculation results.