Wave energy is a viable source of ocean renewable energy and research is being conducted worldwide. The Oscillating Water Column (OWC) device is recognised internationally as one of the most promising types of ocean Wave Energy Converters (WECs). To effectively utilize ocean waves for harvesting more energy, offshore OWC devices need to be deployed in deep–water where waves are more energetic. Therefore, the present paper experimentally investigated the hydrodynamic performance of a 3D offshore–stationary OWC device subjected to a wide range of regular wave conditions of different periods and heights and nonlinear power take–off (PTO) damping conditions simulated by an orifice. The experimental results were also employed to validate a 3D incompressible Computational Fluid Dynamics (CFD) model based on the RANS–VOF approach. It was found that the device capture width ratio decreased as wave height increased, especially for wave frequencies higher than device resonance frequency. However, for low–frequency waves under small PTO damping, there was a noticeable improvement in the device capture width ratio. More importantly, results of this study revealed that even with the changes in the device capture width ratio as wave height doubled, the OWC device could extract more wave energy throughout the whole frequency range tested by a maximum of about 7.7 times, particularly for long waves under small PTO damping. Furthermore, the numerical results from the 3D CFD model were in good agreement with the experiments, while the 2D model provided misleading (overestimating) results for high–frequency waves.