The diffuse upsilon = 0 to 4 levels of the 3p pi (u) e' (3)Delta (u) Rydberg state of O-2 are identified and characterized using (3 + 1) resonance-enhanced multiphoton-ionization spectroscopy. An experimental potential-energy curve for the e(') state, together with the magnitude of its interaction with the A' (3)Delta (u) valence state, are determined semiempirically by analyzing the predissociated spectra using the coupled-channel Schrodinger-equation method. The experimental Rydberg-valence coupling (930 cm(-1)) exceeds significantly the value calculated ab initio by Buenker and Peyerimhoff [Chem. Phys. Lett. 34, 225 (1975)] (< 320 cm(-1)), but is in good agreement with the single-configuration model prediction of Lewis [J. Chem. Phys. 113, 2214 (2000)] (900 cm(-1)).