The spring-like air compressibility effect in oscillating-water-column (OWC) wave energy converters may significantly affect the device’s power performance. Yet, this effect has rarely been accounted for in physical model testing, especially in floating OWCs. Quantifying this effect in the development and analysis of OWCs is crucial for i) making better estimates of their performances and decreasing uncertainties, ii) improving design and testing processes, iii) reducing development costs, and iv) accelerating decision-making on wave energy projects. This paper presents experimental results of a floating coaxial-duct OWC at 1:40-scale. The experimental campaign comprised tests in which the Froude scale was applied to the whole converter structure and, for comparison, tests where the air chamber volume was expanded to comply with the aero-thermodynamic similitude. Tests included regular and irregular waves and various damping conditions. Experimental results show that compressibility effects can increase, decrease, or have neutral contributions to the mean absorbed power, depending on wave frequency. Another effect is the presence of reactive power. The capture width ratio is higher for the compressible experimental configuration in several cases. A new compressibility factor is proposed to aid in designing experimental and numerical campaigns.