The hydrogen carrier gas in an atmospheric pressure epitaxial growth reactor is found to suppress the oxidation of a Si surface intentionally exposed to trace oxygen before epitaxial growth. After an initial delay the oxidation proceeds rapidly to form nanometer size oxide islands at surface coverages in the range of 0.001 to 1 monolayers. A small fraction of the oxide islands nucleate visible pyramidal-shaped defects in the subsequently grown epitaxial layer which enables the interfacial oxidation process to be easily studied. Replacement of the hydrogen ambience with argon leads to more than an order of magnitude increase in initial oxidation rate and a many order of magnitude increase in the density of pyramidal defects for the same trace oxygen exposure. The pyramid density is proportional to the square of the interfacial oxygen density over a wide range of surface oxygen density, 5 x 10(11) cm(-2) to 10(15) cm(-2), and for sample prepared over a wide range of experimental conditions.