In this research, the effect of wave steepness on the hydrodynamic efficiency of an oscillating water column is examined at different chamber lengths and bottom slope angles. Results show that the best device efficiency for low wave frequencies is obtained at high chamber length and for high wave frequencies, the best device performance is achieved at low chamber length. Numerical findings reveal that for high wave steepness, the change in the chamber length has a negligible effect on the device efficiency at low wave frequencies and at high wave frequencies, the best device performance is attained at low chamber length. Results indicate that for the shoreline oscillating water column, the change in the bottom slope angle away from the resonance condition has a negligible effect on the device efficiency and at resonance condition, high bottom slope angle is preferred in order to obtain the best device efficiency. It is found that at high wave steepness, the device efficiency decreases and no specific resonance condition could be observed. It is also shown that the sensitivity of the device efficiency around the optimal device performance to the damping produced by power take-off device decreases as the wave steepness increases.