In the thermal oxidation reaction of Si, point defects (emitted Si atoms and its vacancies) occur owing to oxidation-induced strain. These point defects become not only charge traps that degrade device characteristics but also oxidation reaction sites. However, a reaction rate equation of a model in which point defects are regarded as reaction sites has not yet been proposed. We investigated the O2 pressure dependence of the initial interfacial oxidation rate Ri(0) after the oxidation of an Si(001) surface using real-time Auger electron spectroscopy to establish the reaction rate equation based on the point defect generation model. We found that Ri(0) was proportional to the square root of the O2 pressure, and the initial deceleration of the interfacial oxidation rate was proportional to the O2 pressure. Using the result of the pressure dependence of Ri(0), we derived the reaction rate equation based on the Si oxidation model in which O2 dissociation preferentially occurs at vacancies resulting from Si atom emission at the SiO2/Si(001) interface. The prediction of the O2 pressure dependence of the initial deceleration of the interfacial oxidation rate shows good agreement with the experimental results. In addition, we predicted that there were two kinds of vacancies at the SiO2/Si(001) interface to dissociate the O2 molecules.