The present study investigates the hydrodynamic performance of the two types of oscillating water column wave energy converter devices, namely (i) INP (interface non-piercing) OWC device and (ii) IP (interface piercing) OWC device placed over an undulated seabed in a two-layer fluid system. The boundary element method is used to handle the associated boundary value problems. Major emphasis is given to analyze the efficiency of the OWC devices for various values of incident wave parameters and shape parameters associated with the OWC devices and undulated seabed. Further, the time-domain analysis is provided in which the free surface and interface elevations are demonstrated for different instants of time. It is observed that the efficiency of both the OWC devices associated with the internal wave mode is highly oscillatory and periodic in nature. Further, the resonating pattern in the efficiency curve occurs due to the exchange of wave energy between the surface and internal waves and also due to the multiple resonances and near-resonance interactions of waves with the bottom ripples. Moreover, the results show that the incoming wave energy associated with the internal wave mode is distributed to cover a broadband spectrum, and consequently, the multiple resonances occur in the efficiency curve. In addition, air compressibility significantly influences the hydrodynamic efficiency of the OWC devices.