The OWC spar-buoy is an axisymmetric device consisting basically of a (relatively long) submerged vertical tail tube open at both ends and fixed to a floater that moves essentially in heave. The air flow displaced by the motion of the OWC inner free-surface, relative to the buoy, drives an air turbine. Here, numerical procedures and results are presented for the power output from turbines of different sizes equipping a given OWC spar-buoy in a given offshore wave climate, the rotational speed (maximum allowable blade tip speed of 180 m/s) being optimized for each of the sea states that, together with their frequency of occurrence, characterize the wave climate. Single- and multi-stage Wells turbines and the new biradial impulse turbine were chosen for comparisons. Non-dimensional performance curves of the turbines were obtained from model testing. A stochastic approach was adopted for the hydrodynamic modelling, with air compressibility effects accounted for in a linearized way. The results for the overall performance show that a single-stage Wells turbine would not be a good choice, several stages being required for acceptable performance. The biradial turbine appears as the best choice in terms of performance, with the advantage of substantially smaller rotor diameter.