The intermolecular potential energy surface of O-2-H2O was investigated at ab initio MP2 and MRSDCI levels using the aug-cc-pVTZ basis set. The vibrational levels were evaluated by numerically solving the Schrodinger equations for the nuclear motions with the ab initio potential functions using one- to three- dimensional finite-element methods. On the basis of the calculated partition functions, the equilibrium constant of the complex, K-p was studied. The K-p values at atmospheric temperatures of 200-300 K were found to be 1-2 orders of magnitude less than previous estimates from the harmonic oscillator approximation.