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Dry oxidation of Si (001) beneath a thin epitaxial SrTiO₃ layer has been studied using furnace annealing in flowing oxygen. A 10-nm layer of SrTiO₃ is epitaxially grown on Si with no SiO₂ interlayer. For such a structure, an annealing temperature of 800C was found to be the limiting temperature to prevent silicate formation and disruption of the interface structure. The effect of annealing time on thickness of the SiO₂ layer was investigated. In situ x-ray photoelectron spectroscopy(XPS) and reflection-high-energy electron diffraction(RHEED) were used to ensure that the quality of SrTiO₃ is unchanged after the annealing process. The experimental annealing data is compared with a theoretical oxygen diffusion model based on those due to Deal, Grove and Massoud. The model fits the experimental data well, indicating that oxygen diffusion through the SrTiO₃ layer is not the limiting factor. One can therefore readily control the thickness of the SiO₂ interlayer by simply controlling the annealing time in flowing oxygen.