  The problem of the geometry of interstitial oxygen in silicon is settled by
proper consideration of the quantum delocalization of the oxygen atom around
the bond-center position. The calculated infrared absorption spectrum accounts
for the 517 and 1136 cm$^{-1}$ bands in their position, character, and isotope
shifts. The asymmetric lineshape of the 517 cm$^{-1}$ peak is also well
reproduced. A new, non-infrared-active, symmetric-stretching mode is found at
596 cm$^{-1}$. First-principles calculations are presented supporting the
nontrivial quantum delocalization of the oxygen atom.
