The objective of this paper is to present the wave energy resource assessment having in view the construction of an OWC (oscillating water column) to be integrated into a new breakwater at the mouth of the Douro River in Porto (northern Portugal). The paper starts with the evaluation of the offshore wave energy resource from measured data. This is followed by the wave transformation from offshore conditions to the plant location in about 11 m water-depth. Such transformation was carried out using an inverse-ray refraction model that is described in detail. As expected, the wave power level is lower, whereas the wave energy period is higher, at the plant coastal site as compared with deep water conditions. The average wave direction rotates from approximately NW offshore to nearly West at the plant location, and the directional spread becomes smaller, which are effects of refraction as the waves propagate in waters of decreasing depth.