This paper presehts an experimental and numerical hydrodynamic performance assessment of a 1:50 scale model offshore floating-moored Oscillating Water Column (OWC) wave energy converter. The device is a tension-leg structure with four vertical mooring lines. The performance of the OWC device was investigated for several design parameters including regular and irregular wave conditions of different heights and periods, power take-off (PTO) damping and mooring line pre-tension. A 3D Computational Fluid Dynamics (CFD) model using RANS-VOF approach was constructed and validated against experimental results for regular waves showing good agreement. It was found that the hydrodynamic efficiency of the floating-moored OWC device follows the same general trend as a fully-constrained (fixed) model, but the addition of surge motion in the floating device improved the energy production efficiency over a broader bandwidth around the chamber resonance. Increasing the incoming wave height resulted in a higher efficiency for low-frequency waves, but noticeable reductions in the efficiency were observed in the intermediate-and high-frequency zones. The effectiveness of utilizing offshore OWC devices in deep-water was demonstrated by increasing the extracted energy by a maximum of 7.7 times and 5.7 times when regular and irregular wave heights were doubled, respectively. Decreasing the mooring line pre-tension slightly increases the energy extraction efficiency in the intermediate-frequency zone.