The Oscillating Water Column (OWC), one type of the wave energy converter (WEC), has been the focus of many researches of marine energy because of its simplicity. The efficiency of an OWC is apparently affected by the incident wave conditions. This paper studies the effect of the incident wave conditions on the efficiency of an OWC using experiments with a model whose geometry is similar to that of the LIMPET. Under different incident wave conditions, we perform experiments with an outlet orifice of two diameters to simulate different pneumatic damping levels of the air turbine. Time-series images of the water level inside the OWC are first obtained by high-speed imaging, and then processed and analysed with a procedure to find the position and velocity of the water level. The capture factor of the OWC is related to the water level’s kinematic characteristics using a simple hydrodynamic theory. The results indicate that the OWC’s efficiency is strongly related to the incident wave conditions as well as the pneumatic damping levels.