We investigated the oxidation reaction of the O-2 molecule at the Si(100)-SiO2 interface by using a constrained ab initio molecular dynamics approach. To represent the Si(100)-SiO2 interface, we adopted several model interfaces whose structural properties are consistent with atomic-scale information obtained from a variety of experimental probes. We addressed the oxidation reaction by sampling different reaction pathways of the O-2 molecule at the interface. 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, regardless of the spin state of the O-2 molecule. Our study suggests a picture of the silicon oxidation process entirely based on diffusive processes.