Amongst the different wave energy converters (WECs), Oscillating Water Column (OWC) has aroused the interest of the scientific community and many companies. The performance of this type of systems depends on several parameters like incident wave conditions, geometry of the chamber structure, control system or the operation characteristic curves of the air turbine. A real case study of a fixed OWC is the breakwater of Mutriku harbour (Bay of Biscay), composed by 16 chambers with a self-rectifying air turbine of Wells type. In the present work, we will focus on the hydrodynamic efficiency of the chamber geometry of this facility. According to available information of the site, a tide level and a wave height have been selected. The pneumatic power as a function of the generated air pressure and flow rate will be studied according to the incident potential and kinetic energy of the incoming waves at several operation points of the Power Take Off (PTO) turbine defined by the corresponding damping state. Experiments using a scale model have been performed in a 2D wave flume. Different damping conditions (orifice plate diameters) have been studied for different regular waves with a broad range of periods, having identified the optimal conditions for energy conversion. The work is a first milestone of a wider job that will contribute to the knowledge of this power plant and its most appropriate operating conditions, to help in the decisions to be undertaken in any future action on this plant, or, in a wider scope, to any new projected onshore OWC power plant.