The primary efficiency of the Oscillating Water Column (OWC) Wave Energy Converter (WEC) device in the stepped sea bottom condition is investigated using both experimental and numerical approaches. Wave flume tests were undertaken to investigate the hydrodynamic behaviour of the device in regular waves. A 2D numerical model was developed in the open source computational fluid dynamics (CFD) software package OpenFOAM implementing the fully non-linear Reynolds Averaged Navier-Stokes (RANS) equations to simulate the wave power absorption and wave structure interactions. The numerical results have been validated against the experimental data. The influence of the wave characteristic as well as damping of the power take-off unit on the performance of the device, wave reflection coefficient and the energy dissipation rate is evaluated using the results obtained from the numerical simulations. Furthermore, as was proved in previous studies that the application of the stepped sea bottom condition might increase the efficiency of the OWC devices, the CFD simulation results have been implemented to present and discuss about the power absorption mechanism as well as the flow pattern characteristics in the vicinity area of the step.