The movement of the free surface inside a surface piercing cylindrical duct, simulating a floating OWC in regular waves, was analysed experimentally and by numerical simulation, and the results were compared with an analytically obtained solution. This provided insight into the influence of each of the terms in the equations that govern the dynamics of the system, especially the hydrodynamic and damping coefficients. The experimental values show that the viscous damping due to real fluid effects is important, at least at the scale of the model. The prediction of the natural frequencies was found to be accurate. The measured dynamic response of the system agreed fairly well with the predictions. A detailed analysis of the free surface shape for different resonant frequencies showed that the sloshing modes are present and their shape can be determined from numerical simulation. However, they are significant for frequency much higher than the range of frequencies of interest for the OWC applications where the column is narrow with respect to the width. The consideration of the wall thickness, water depth and the order of the numerical solution has significant importance in the radiation damping.