•We implement and validate a RANS–VOF model of an OWC wave energy converter.•We study the OWC performances for different wave conditions and damping values.•Damping is the main factor of OWC performances, closely followed by period.•There is an optimum damping for each wave condition.•We develop a procedure to determine the optimum turbine-chamber coupling. The performance of an oscillating water column (OWC) wave energy converter depends on many factors, among which the incident wave conditions, the tidal level or the coupling between the chamber and the air turbine. In this work a 2D numerical model based on the RANS equations and the VOF surface capturing scheme (RANS–VOF) is implemented in order to study the optimum turbine-chamber coupling for a given OWC. The model represents a numerical wave flume where the OWC is tested under regular and irregular waves and for different damping coefficients, i.e., turbines of different characteristics. First, the numerical model is validated under regular and irregular waves using results from physical model tests. Excellent agreement is obtained between both models, physical and numerical. After the validation, an extensive campaign of computational tests is carried out, studying the performance of the OWC under nine different damping coefficients. The model developed allows, first, to quantify the relevance of the damping coefficient and wave conditions on the performance of an OWC chamber; and second, to define the damping condition which maximizes that performance, determining the characteristics that a turbine must meet to achieve the optimum coupling. In this manner this work contributes to the development of high performance OWCs.