A three dimensional, fully nonlinear dual phase numerical wave tank model is created using the open source software OpenFOAM. We incorporate a range of varied Oscillating Water Column (OWC) chamber geometries into the model to investigate the influence of chamber shape on the efficiency response of the OWC. The model is validated through comparison with previously published experimentally obtained chamber free surface heights and chamber pressure data sets. A number of simulations are performed over a range of wave periods for each of the chamber geometries investigated. Then, we generate the efficiency response curves for each of the chamber geometries. The results show that the chamber geometry has a significant influence on the OWC efficiency especially in terms of the optimal performance frequency bandwidths. It is observed that the traditional rectangular box shape OWC chamber seems to be the optimal geometry both in terms of maximum efficiency achieved and in terms of the broadest bandwidth to achieve wave energy extraction.