Using constrained ab initio molecular dynamics, we investigate the reaction of the O-2 molecule at the Si(100)-SiO2 interface during Si oxidation. The reaction proceeds sequentially through the incorporation of the O-2 molecule in a Si-Si bond and the dissociation of the resulting network O-2 species. The oxidation reaction occurs nearly spontaneously and is exothermic, irrespective of the O-2 spin state or of the amount of excess negative charge available at the interface. The reaction evolves through the generation of network coordination defects associated with charge transfers. Our investigation suggests that the Si oxidation process is fully governed by diffusion.