The oscillating water column, which has a pneumatic section, can convert wave energy to air pressure, and thus, it rotates the turbine. The system has an inherent weakness of having low efficiency. Typically, the small Wells or axial turbines without any modifications have an efficiency of less than 40% under good sea wave condition which is a drawback and considered as less efficient to extract energy from the oceans. In this paper, the performance of the turbine is studied and analyzed by a computational fluid dynamics technique. ANSYS commercial code is used to discretize the mass, momentum, energy equations, and the turbine efficiency was evaluated. Different guide vane angles and turbine diameters were considered for the simulations. The modified turbines are simulated to operate in various rotational speeds which are similar to an actual working of oscillating water column. This helps to find a suitable operational range for the proposed turbine for energy extraction. It was found that the bigger turbines are expected to produce relatively higher efficiencies at lower speeds and lower pressures.