In this paper, we discuss the effects of geometry and dimensions of an oscillating water column energy converter chamber on the efficiency of this device. The main purpose of this study is to optimize the geometry and dimensions of the energy converter to get the maximum power available in a progressive wave with constant period and wavelength. Modeling and numerical simulation are performed by using the commercial software ANSYS- ICEMCFD & CFX. The numerical wave tank used in this model is assumed to be equipped with a Stokes second order wave generator type. The fluid flow is set to be biphasic (air/water), two-dimensional and k-ɛ turbulent model. Since the study is focused on the temporal variation of the pressure, the turbine power extraction is modeled by a vent that shape and location can be variables. The simulation results shows that the size of the chamber, the immersion depth of the front wall of the device and its orientation versus the flow direction have a very significant impact on the performance of the device.