This paper uses a fully nonlinear model to comprehensively explore the coupled effects caused by the system integration between an offshore wind turbine and a heaving buoy wave energy converter. Parameters of the study include the pitch motion of wind turbine, the tension on the mooring line, the contact pressure between these two devices, and the energy absorbed by power take-off (PTO) system. Results demonstrated that the buoy stabilises/reduces the pitch motion of wind turbine when its metacentric height is positive. A buoy with a negative metacentric height will increase the pitch amplitude of wind turbine and the tension of the mooring line, which is undesirable. The relative vertical motion between heaving buoy and spar could buffer their maximum contact pressure. The Coulomb PTO could offer a higher peak output power of WEC than the linear PTO. The relationship between contact pressure and WEC peak power is quantified to inform the PTO design. The wind/wave device is evaluated at a representative site with suitable wind and wave conditions off the US West Coast. The WEC does significantly reduce the wind turbine pitch motion by at least 60%/50% for the modelled average/max wave conditions with increased power production (14%/80%).