  Oxygen migration in silicon corresponds to an apparently simple jump between
neighboring bridge sites. Yet, extensive theoretical calculations have so far
produced conflicting results and have failed to provide a satisfactory account
of the observed $2.5$ eV activation energy. We report a comprehensive set of
first-principles calculations that demonstrate that the seemingly simple oxygen
jump is actually a complex process involving coupled barriers and can be
properly described quantitatively in terms of an energy hypersurface with a
``saddle ridge'' and an activation energy of $\sim 2.5 $ eV. Earlier
calculations correspond to different points or lines on this hypersurface.
