Like other color centers (VK, H) the O-2-center in alkali halides has a strong molecular character. It is shown that methods similar to those used in the theory of transition metal salts lead to an understanding of the electronic ground state and g-factor of the O-2-center. It is necessary to invoke a weak covalent bonding with the neighbouring ions. This bonding manifests itself in the crystal field splitting, E. P. R. linewith and the reduction of orbital angular momentum matrix elements. The H. F. S. of the (16O-17O)- center has been measured. The experimental results can be understood by means of a symmetry analysis of molecular hyperfine coupling. Instead of the single parameter < 1/r3 > of the Hartree-Fock approximation, four parameters are needed for a complete description of the H. F. S. of a molecule in a Π state