The pressure and SiO-incorporation effect on the temperature dependence of dynamical properties for silicon oxide (SiO2)1-x(SiO)x is theoretically studied by first-principles molecular dynamics. It is found that the incorporation of SiO enhances the selfdiffusions even under high pressure. The SiO effect on Si selfdiffusion is however reduced by the pressure at temperature lower than 4000 K, and the amount of reduction is larger for lower temperature, while the effect on O selfdiffusion is hardly reduced, being independent of the temperature. Such a difference is thought to come from the difference in selfdiffusion mechanisms between Si and O. It is indicated that the incorporated SiO acts as combined SiO interstitials rather than separate Si interstitials or O vacancies. This also suggests that the oxide viscous flow mechanism is a promising candidate for the origin of the Si missing in oxidizing Si nanopillars.