This paper presents the development of a Computational Fluid Dynamics (CFD) model for a free heaving Oscillating Water Column (OWC) spar buoy with non-linear Power Take Off (PTO). Firstly, a freely heaving barge was applied to a 2D Numerical Wave Tank (NWT), used to validate a 1 Degree Of Freedom (DOF) modelling methodology. Multiple sets of regular waves were used to assess the heave response compared to previous experimental and numerical studies. In parallel, the NWT was extended to 3D where analyses of incident waves have been conducted to ensure accurate waves are portrayed. A PTO boundary condition was created to replicate a non-linear impulse turbine, typically simulated by an orifice plate in scaled models. The PTO boundary was compared and validated using experimental data. Finally, a comprehensive system comprising of the 3D NWT, 1DOF set-up and non-linear PTO allowed the development of a heave-only OWC spar buoy model with a non-linear PTO. Experiments completed by UCC MaREI centre in LIR-NOTF ocean wave basin under FP7 MARINET project is detailed and used to validate the comprehensive model. A range of regular waves were applied and responses of heave and chamber pressures were compared to experimental data, which showed excellent correlation.