This paper summarizes the theory behind the modeling that was performed to incorporate an oscillating-water-column type Wave energy Converter (WEC) into the WindFloat hull. The WindFloat is a floating structure supporting a very large (>5MW) wind turbine. By adding a WEC to the structure, the overall economic cost of the project can be improved by sharing both mooring and power infrastructure. A numerical model was developed using the diffraction-radiation code WAMIT and assuming as PTO equipment, a generic wells turbine. It is important to model the turbine accurately, to understand the power capacity of the device. Details on the modeling of the system are discussed and numerical results and compared against experiments as a validation of the model. The effect of coupling between the floating foundation of the WindFloat and the OWC is investigated thoroughly.