This paper provides a thorough examination and discussion of a design process developed to improve the hydrodynamic performance of an asymmetrical offshore Oscillating Water Column (OWC) Wave Energy Converter (WEC). The resulting WEC geometry was based on a column-stabilised semi-submersible platform where an asymmetrical OWC chamber was integrated into the moonpool of the platform, resembling both, a purpose built WEC, or an existing offshore structure retrofitted for wave energy conversion. The performance of a 1:36 scale model of the OWC WEC was designed using a validated computational fluid dynamics (CFD) method and experimentally tested to evaluate the effect of the external support structure on the hydrodynamic performance of the device. Detailed analysis included physical and numerical decay tests to quantify the natural period of the OWC moonpool, wave and OWC-structure interactions, turbine damping coefficients and hydrodynamic capture width ratios. The obtained results revealed that the addition of the external support structure improved the OWC Capture Width Ratio (CWR) from 0.849 at kd ∼ 2.53 to 1.541 at kd ∼ 2.99 (81.5% increase). It was also observed that the external support structure shifted the peak performance towards higher frequency waves.