Understanding the hydrodynamic performance of an Oscillating Water Column (OWC) wave energy converter is essential for further improving the device efficiency. Most experimental measurements are performed at small scales where air compressibility can be ignored. In this paper, scaling and air compressibility effects on the performance of an offshore stationary OWC wave energy converter are studied in a 3D numerical wave tank. A Computational Fluid Dynamics (CFD) model based on the RANS equations and the VOF surface capturing scheme is developed and validated against physical measurements of a 1:50 model-scale OWC. Following the validation stage, the CFD model is utilized to investigate scaling and air compressibility effects on the performance of an OWC by comparing five different scales from 1:50 up to full-scale and modelling air as both compressible and incompressible. Other parameters that are considered include the impact of varying the power take-off (PTO) damping, wave height and period, underwater geometry, and the effect of air chamber height. Results reveal that at full-scale air compressibility is important and can induce about 12% reduction in the maximum efficiency predicted at model-scale under regular waves at the resonant frequency and the optimum PTO damping. Air compressibility effects slightly reduce as the extracted pneumatic power increases. In order to consider full-scale air compressibility effects while testing a small OWC model scaled using Froude’s similitude law, the full-scale air chamber volume is best scaled with the square of the scale factor rather than cubic to predict the device performance with errors of less than 1.2%. In this manner, this work contributes to the full-scale development of OWCs by better understanding the differences from small physical model-scales.