The beneficial effects of designing an oscillating water column (OWC) wave energy converter (WEC) by associating the hydrodynamics of the oscillating water masses to a dual-mass systems are described. The primary efficiency of a simple one degree of freedom WEC as well as a dual-mass WEC concept is derived analytically for shallow water waves. It is observed that substantial improvements in primary efficiency bandwidth could be reached by the application of the dual-mass concept. The primary efficiency of an OWC device deployed on stepped sea bottom is subsequently investigated, based on both numerical and experimental approaches, as an example of a dual-mass system. In the numerical approach, the performance of the device is estimated in both regular and random waves. A comprehensive experimental campaign is carried out to validate the numerical results as well as to investigate the efficiency improvements achieved by adding the step in the sea bottom for various cases of incident wave heights and turbine damping. It is observed that significant improvements in the performance of the OWC device in the stepped sea bottom condition can be reached, which corresponds to the implementation of the dual-mass system in the conceptual design of this device.