The oscillating water column (OWC) responses within forward, central, and aft located moonpools for a fixed rectangular vessel are studied numerically under regular head wave conditions using RANS-based CFD calculations. The free surface elevation of the confined water inside the moonpool for the piston mode frequencies is studied independently for the forward, central, and aft moonpools, showing a strong dependence on location, draft, and incident wave frequency. The relative response is largely magnified for wave frequencies around the resonant range. It is observed that the free surface response amplitude is significantly higher in the forward moonpool location, and the results are in well comparison with experiments performed in the towing tank. The variation of dynamic pressure and its influence on the moonpool responses is investigated. The dynamic pressure and its influence on the moonpool responses is investigated to understand the position-specific fluctuations, which showed the presence of harmonics. The phase difference between the water column oscillations within the moonpool and the propagating wave in the domain depends on the its position. Finally, the effect of the moonpool in modifying the pressure field is studied by comparison with a vessel without moonpool for a common incident wave frequency.