The performance evaluation of a wave energy converter in wave channel is influenced by the hydrodynamic effects caused by the near presence of the side walls. Since this phenomenon is not observed in the open ocean, it is important to assess the walls influence in the converter dynamics when analysing experimental results. This paper studies the dynamics and power extraction of an axisymmetric floating oscillating water column (OWC) device, the Spar-buoy OWC, using experimental data obtained in a wave channel. A two heaving body model (spar-buoy and OWC) based on linear forces is formulated in the frequency domain. Linear hydrodynamic coefficients are obtained from a boundary integral equation method. The presence of the channel side walls is simulated approximately by a periodic array of devices, and alternatively by two finite-length walls. Linearized drag forces are derived from small-scale model tests. Power extraction results are presented for regular and irregular waves. The numerical simulations show that the wall effect may amplify the power capture up to a maximum of 15% for regular waves and 10% for irregular wave conditions, for a channel-width-to-device-diameter ratio equal to 5.25.