An atomic-scale description is provided for the long-range oxygen migration through the disordered SiO2 oxide during silicon oxidation. First-principles calculations, classical molecular dynamics, and Monte Carlo simulations are used in sequence to span the relevant length and time scales. The O-2 molecule is firmly identified as the transported oxygen species and is found to percolate through interstices without exchanging oxygen atoms with the network. The interstitial network for O-2 diffusion is statistically described in terms of its potential energy landscape and connectivity. The associated activation energy is found in agreement with experimental values.