The sustainable development of the offshore wind and wave energy sectors requires optimising the exploitation of the resources, and it is in relation to this and the shared challenge for both industries to reduce their costs that the option of integrating offshore wind and wave energy arose during the past decade. The relevant aspects of this integration are addressed in this work, and in particular the evaluation of the Wave Energy Converter (WEC) sub-system of a hybrid wind-wave energy converter: the state of the art of combined technologies; the definition of a novel hybrid prototype, based on a preliminary feasibility analysis of a conceptual proposal; and the evaluation of a simplified version of this prototype by means of physical and numerical modelling as a mean to set the reference and define new tools and methods for future evaluation and optimisation of the prototype. Because of the novelty of combined wave and offshore wind systems, fundamental knowledge was lacking as, for example a comprehensive review and classification, which was published as a journal paper framed in the present work. In particular, the core of this PhD thesis deals with the WEC sub-system of a hybrid device that integrates an Oscillating Water Column (OWC) device into the typical monopile substructure of an offshore wind turbine. A new prototype of the hybrid energy converter has been proposed, and a patent application was filled. Furthermore, an experimental set-up was designed, built and tested at a wave flume. On the basis of this experimental campaign the performance of the device is analysed. Finally, a full 3D-numerical mirror of the experimental set-up, including the hybrid energy converter, is defined and validated, and the flume enclosure effects studied for regular waves.