The Oscillating Water Column (OWC) is one of the most popular devices for harvesting ocean wave energy because it is so easy to construct and operate. The novelty of this study is to investigate the effect of triangular shape leap walls on the hydrodynamic performance of oscillating water column (OWC) devices by changing the relative opening and orifice ratio using a numerical simulation approach. This investigation is performed in a 3-D numerical wave tank using CFD code ANSYS FLUENT software version 18.1. Navier–Stokes equations are discretized by the Finite Volume Method (FVM). The numerical simulation is carried out under different wave steepness conditions, and the model is validated by comparing the present numerical results with the published experimental data. The appropriate waves are produced using the inlet velocity approach. In terms of orifice ratios (τ) and relative opening (α) OWC performance has been investigated. Also, It has been observed that hydrodynamic efficiency initially increases between the orifice ratio τ = 0.4–0.8% and then decreases between τ = 0.8–1.3% for all considerations of relative opening (α). The computational study helps better understand the intricate physics of OWC devices and gives prior knowledge of the performance of OWC before the real-life experiment. The appropriate waves are produced using the inlet velocity approach. In terms of orifice ratios (τ) and relative opening (α), OWC performance has been investigated.