Utilization of Boundary Element Method (BEM) based on linear potential flow for modelling Oscillating Water Column (OWC) devices has gained popularity in the last two decades. The commercial BEM solver WAMIT has been used widely for modelling OWCs and validated using experimental modelling (Delauré et. al. 2003, Bingham et. al. 2015, Faÿ 2020). The open-source BEM solver NEMOH has however been mostly ineffective in modelling OWCs since the main approach adopted previously modelled the imaginary piston as a thin disk. In this research, the multi-body interaction problem has been adopted in modelling a box-type and bottom-detached OWC device in NEMOH, where the imaginary piston has been modelled to the length of the internal water column (Penalba et.al. 2017) and compared with experimental data. A further comparison is drawn with the numerical method of Computational Fluid Dynamics (CFD) , which has shown to be accurate for modelling OWC devices (Simonetti et. al. 2015), yet requires significantly higher computational resources than BEM. A two-dimensional CFD numerical wave tank, developed generating and absorbing waves with the waves2Foam toolbox (Jacobsen et al., 2012) of the open-source package OpenFOAM, is used for comparative purposes.