The diffusion of oxygen in silicon was modeled to result from the diffusion of dissolved silicon monoxide. The SiO molecule dissolved in the largest space in the diamond lattice of silicon, oriented in a 〈111〉 direction, with the oxygen lightly bonded to a network silicon atom. This configuration is consistent with infrared spectra, dichroism of infrared spectral lines, and internal friction of oxygen in silicon. The anomalous rapid diffusion of oxygen below 700 °C could result from the diffusion of molecular water or oxygen in silicon, but more evidence is needed to test these possibilities. Diffusion and exchange of silicon tracer in SiO with lattice silicon can possibly explain tracer diffusion of silicon in silicon.