Air compressibility is often neglected in experimental work due to practical difficulties, even though it is known to affect the performance of OWC wave energy converters. The key question, of course, is to what extent. In this work the impact of air compressibility on the capture width ratio is thoroughly quantified by means of a comprehensive experimental campaign, with no fewer than 330 tests encompassing a wide range of wave conditions and levels of turbine-induced damping, and two experimental set-ups: one designed to account for air compressibility, the other to neglect it. This approach is complemented with the use of the RANS-based CFD model OpenFOAM® to calibrate the pressure-vs-flowrate curves, which enables the flowrate to be determined based on the pressure drop measurements from the physical model. We find that the errors that derive from disregarding air compressibility may lead to either under- or over-predictions of power output, and are highly dependent on the operating conditions, more specifically the wave conditions (sea state) and turbine-induced damping.