Oscillating water columns (OWCs) are most widely used in coastal wave energy conversion. The air duct opens into the atmosphere through the air turbine, which is the power take-off device, and this results in a pressure drop across the air chamber. However, because of the complex configure of the impulse turbine and its high rotation speed, it is difficult to install it in the experimental simulator and numerical model. Therefore, the turbine damping effects on the operation of the OWC air chamber are induced to predict its performance more accurately. Orifice plates are used as a substitute for the impulse turbine as it generates a similar pressure drop and power output; the experimental and numerical pressure drops and output powers are compared. A 3D numerical wave tank based on the two-phase VOF model is established using the commercial CFD code Fluent, which can predict air flow and pressure variations in the chamber and duct. Water surface elevations, air flow velocity and pressure variation inside the chamber with the orifice plate are studied numerically, and validated by the corresponding experimental data. The air chamber of the Yongsoo OWC pilot plant is used as the engineering project case. The operating performance of the air chamber installed with a 0.428D orifice plate as the substitute for the designed impulse turbine is computed and analyzed. It is found that the turbine damping effects will cause around 30% reduction in the peak values of the pneumatic energy output of the OWC air chamber in the resonant wave domain.