Quantum mechanical ab initio calculations have been used to determine stable complexes formed between the atomic oxygen radical anion, O −, and the first period hydrides, XH n =LiH, BeH 2, BH 3, CH 4, NH 3, H 2O, and HF. The calculations were performed using second order Möller Plesset approximations with basis sets incorporating polarization and diffuse functions. In all, 29 stable complexes have been identified. The structures of these complexes suggest they should be grouped into six families which may be described in a compact way as: (O −,H XH n−1 ), (HO −,H XH n−2 ), (HO XH n−1 ) −, (H 2,O XH n−2 ) −, (O XH n ) −, and (H 2O, XH n−2 ) −. The stability of the complexes, as measured by the change in standard free energy for the reaction XH n +O −→ complex, is for all families a smooth function of the atomic number of X.