Existing theoretical models are unable to account for a number of important optical properties of oxygen in GaP. In this paper an energy-level structure is proposed which explains in detail the optical spectra in terms of spin- and k-conserving one-electron transitions. This scheme is based on spin multiplets formed from the neighboring gallium orbitals, which interact only weakly with a neutral O atom. Its striking success exposes limitations of more conventional single-particle theories. © 1982 The American Physical Society.