The requirement for increasingly thin (<50 Angstrom) insulating oxide layers in silicon-based electronic devices highlights the importance of characterizing the Si-SiO2 interface structure at the atomic scale. Such a characterization relies to a large extent on an understanding of the atomic-scale mechanisms that govern the oxidation process. The widely used Deal-Grove model invokes a two-step process in which oxygen first diffuses through the amorphous oxide network before attacking the silicon substrate, resulting in the formation of new oxide at the buried interface(1). But it remains unclear how such a process can yield the observed near-perfect interface(2-12). Here we use first-principles molecular dynamics(13-15) to generate a model interface structure by simulating the oxidation of three silicon layers. The resulting structure reveals an unexpected excess of silicon atoms at the interface, yet shows no bonding defects. Changes in the bonding network near the interface occur during the simulation via transient exchange events wherein oxygen atoms are momentarily bonded to three silicon atoms-this mechanism enables the interface to evolve without leaving dangling bonds.