The oscillating water column is the most widely used wave energy converting technology for onshore applications. In the previous studies, the performances of the chamber and the turbine were investigated separately, or some substitutions were employed to induce the damping effects. In this study, an OWC model employing the cubic chamber and the impulse turbine is studied experimentally in a wave flume. The model turbine operates under the free-spinning mode with the resistance connecting to the electrical generator as the external loads. Regular wave conditions are applied to generate different wave scenarios to investigate the hydrodynamic and aerodynamic performance of the chamber-turbine system of the OWC model. The primary-stage, secondary-stage, and overall efficiencies under the free-spinning mode for various wave conditions and load coefficients are presented. The peak efficiencies are compared with those under the fixed rotation-speed mode. Experimental results indicate that the chamber length should be designed according to the local predominant wave climate, and the proper external resistances should be chosen to enhance its overall efficiency.