The intra-annual variability in the wave resource is often disregarded when analysing the performance of wave energy converters (WECs), despite the fact that this variability is substantial in the majority of the areas of interest for the development of wave energy. The objective of this work is to analyse and quantify the intra-annual variability in the performance of oscillating water column (OWC) WECs through a case study in Galicia (NW Spain). To this end a three-step methodology which combines numerical and experimental modelling is followed: (i) intra-annual wave energy resource matrices are determined numerically through a high-resolution procedure; (ii) efficiency matrices of the device are determined by means of physical modelling, considering the influence of air compressibility and different turbine specifications represented through different values of turbine-induced damping; and (iii) finally, intra-annual energy capture matrices are calculated by combining the resource and efficiency matrices. It is found that the intra-annual variability in the energy capture of an OWC converter is significant, over 20% in the case study considered, albeit slightly smaller than that of the wave energy resource itself. The turbine-induced damping exerts a modulating effect over the variability in the intra-annual captured energy. Furthermore, the optimum damping which maximises the performance of the OWC converter varies from month to month.