The oscillating-water-column (OWC) wave energy converter with air turbine has been object of extensive development. The spring-like effect of air compressibility in the chamber is related to the density–pressure relationship. It is known to significantly affect the power performance of the full-sized converter, and is rarely accounted for in model testing. A method is presented to correct results from physical model testing for effects of air compressibility. It combines linear theory in the frequency domain and hydrodynamic coefficients, together with air thermodynamics. A new concept of linear turbine simulator aerodynamically equivalent to the orifice simulator is introduced in the theory. Corrections for non-linear real-fluid effects may be accounted for from comparisons with data from wave tank testing. The method was validated in a case study involving published data from OWC model testing with regular waves of different periods and amplitudes in a wave flume. Theoretically based corrections were found to well predict the air compressibility effects upon power performance. Air compressibility may negatively or positively affect power conversion depending on whether the wave period is larger or smaller than a critical value accurately predicted by the theory.