Based on an efficient potential flow wave model to simulate the oscillating water column (OWC) wave fields, this research further develops the solutions using higher index terms in the wave potential. This effort together with a linear damping correction significantly improves the free surface elevation correlation between the potential model predictions and the model test measurements. In addition, a method to establish conservation of energy within the incident, diffracted, and radiated wave energy flux is introduced. The method uses the gradient of wave phase to efficiently determine the propagating direction of a small element of wave in the wave field. By integrating over the control boundary, this research provides the energy balance for diffraction-only case and diffraction-radiation combined case. Discussions about the influence of different parameters on the energy extraction from the wave field demonstrate the effective application of the method in optimizing wave energy converters utilizing oscillations induced by 3-D wave field.