The rapid development of the oscillating water column (OWC) wave energy converter proposes a demand to predict its overall performance accurately. Traditional experimental and numerical studies only employed the orifice or porous media as the power take-off substitutes, causing an inaccurate evaluation of the OWC performance. In this study, an impulse turbine model was used directly in a generic OWC model for the secondary-stage energy conversion. The wave-flume tests were conducted under irregular wave conditions and the rotation speeds of the turbine were fixed. Typical time histories of some performance parameters were presented. The primary-stage and secondary-stage efficiencies were analyzed under various test conditions. It was found that the distribution characteristics of the overall efficiency of the chamber-turbine system is significantly determined by the secondary-stage efficiency. The different distribution characteristics of the efficiencies for the regular and irregular wave conditions indicated that the irregular wave tests are preferred for the shape parameter and operating mode design of the OWC plant.