Multi-functional platform is a promising way to enhance the economic power production from multiple renewable energy sources. This paper investigates numerically and experimentally the hydrodynamic performance of an oscillating water column (OWC) wave energy converter (WEC), integrated into a monopile-mounted offshore wind turbine (OWT). Based on linear potential flow theory, a 3D time-domain numerical model was developed, based on the higher-order boundary element method, to investigate the coupled hydrodynamic response of a cylindrical-type OWC device. A nonlinear pneumatic model was utilized to simulate the turbine damping. Experiments on the integrated system were carried out in a wave flume at Dalian University of Technology. The numerical results agree well with the experimental studies, including i) the surface elevation and air pressure inside the chamber, ii) wave pressure on the OWT monopile and iii) hydrodynamic efficiency. Furthermore, the effects of the OWC damping and wave steepness on the OWC-OWT system were investigated. It was found that the introduction of the OWC can significantly reduce the horizontal force and overturning moment on the OWT monopile, and that the wave steepness has a significant influence on the OWC efficiency, especially at resonance.