Ab initio calculations are made to examine theoretically the possibility of stable existence of [OH-;e(+)] system. Diffuse functions are added to the conventional 6-31G basis set, considering the wide spread of positron orbital. Moreover, the Moller-Plesset perturbation of the second order is calculated to take the electron correlation into account. These two improvements are found to be very effective fdr the stable existence of the system. The positron affinity of OH- is computed to be 4.9 eV, and the binding energy of positronium to OH as 0.7 eV which is in good agreement with experimental estimate.