One of the most promising wave energy converter type is Oscillating Water Column (OWC) system. Fluctuation amounts and the motion behavior of the water column inside the chamber are very important parameters effecting the energy extraction. Therefore, predicting these parameters with respect to varying wave characteristics and geometric design parameters is of great importance. In this study, physical experiments are conducted for a bottom-fixed OWC system with seven different sizes of opening heights under various regular wave series. Average fluctuations inside the chamber are measured. It is found that there is a critical relative opening height ratio (alpha) that makes the fluctuations maximum regardless of wave parameters used. Exponential and linear relationships are found between average fluctuations and dimensionless parameters 'dimensionless wave frequency' and 'captured wavelength' defined, respectively. Mathematical models are developed to predict water column fluctuations under varying relative opening heights and wave parameters. The results of mathematical models indicated good aggrement with experimental data. Also chamber water surface profiles are observed and related to defined dimensionless wave parameters. Another factor (named as excessive harmful energy) is determined which also induces sloshing motion inside the chamber after the critical ratio value cc is exceeded. It is found that under all wave series, the highest relative average column water surface fluctuations occur at relative opening height a is equal to 0.67 which is a unique value. It can be concluded that mathematical models can be used to estimate water column fluctuations from relative opening height and wave parameter data in the chamber.