We present full ab initial calculations of both molecular and atomic oxygen reacting with hydrogen passivated Si(100) 2 x 1 reconstructed surface. The calculations are based on the density functional theory via a pseudopotential method. Preliminary results reported concern the search for the oxygen preferential setting on the surface. For molecular oxygen only physisorbed states are found. For atomic oxygen, oxidation reaction occurs and we find two preferential sites for the oxygen. In the first case we show how Si-H bonds on the hydrogenated surface dimers are broken. The second case is a direct barrierless and destructive subsurface oxidation.