Water surface fluctuations inside the chamber of an Oscillating water column (OWC) type wave energy converter (WEC) are very important since they are the conveying processes in conversion of wave energy to electricity. In this study, a mathematical vibration model is developed to estimate the water surface average fluctuations in the chamber and the related phase angles. Resistive forces against the motion of the water column in the chamber are represented by introduced damping coefficient in the equations and determined experimentally by a novel way that is not present in the literature. A particular relative opening height of the chamber is revealed that provides minimum damping which in turn maximizes the highest average chamber water surface fluctuation value regardless of the incident wave parameters. A mathematical vibration model is developed to simulate the water surface fluctuations inside the chamber under different wave conditions and chamber opening heights. Physical experiments were performed to validate the mathematical vibration model results. It is observed that a good agreement exists between the physical experimental data and the mathematical vibration model results. (C) 2019 Elsevier Ltd. All rights reserved.