In this work, the maximum theoretical hydrodynamic efficiency of an onshore Oscillating Water Column (OWC) device is investigated in relation to the incidence angle of the wave and the configuration of the chamber. Linearized water wave theory is used, and the full solution of the associated boundary value problem (BVP) is achieved by using the simple matched eigenfunction expansion method (EEM) and the boundary element method (BEM) with second-order elements. This study is novel in using these techniques to address the influence of oblique water waves on the efficiency of an OWC with a thick front barrier. The hydrodynamic efficiency is analyzed for different directions of wave incidence and OWC chamber geometrical configurations. The semi-analytical and numerical approaches were found to be in good agreement with cases published in the specialized literature. The results show that both a thick front wall and a large incidence angle of the wave can significantly narrow the hydrodynamic efficiency band and modify the resonant frequency.