In an OWC (oscillating water column) wave energy converter the damping exerted by the turbine on the movements of the water column is one of the main factors, if not the main, affecting the power output. In this work the effects of the turbine-induced damping and the variation in the tidal level on the efficiency of the converter are investigated by means of a novel approach. PIV (particle imaging velocimetry) is used to determine the characteristics of the flow (the velocity and vorticity fields, and the kinetic energy) through a phase-averaging procedure. Then, Reynolds decomposition is applied to separate the velocity fluctuations for each test in order to estimate the turbulent kinetic energy. On this basis, we establish the relevance of the different factors—damping, tidal level and wave conditions—to the hydrodynamic performance of the OWC. We find that the turbine-induced damping is the factor that plays the main role: it affects the hydrodynamic behaviour of the chamber and thereby determines the amount of energy that the OWC is able to capture. In addition, the lip is found to be the critical element of the OWC chamber from the point of view of the structural design.