A comprehensive understanding of the flow within an oscillating water column (OWC) is essential to improving the efficiency of the underwater geometry of this type of wave energy converter. This study aims to investigate the impact of the sidewalls on the flow and the changes in flow across the device. Scale model experiments were performed on a forward facing bent duct OWC to generate two-dimensional (2D) particle image velocimetry (PIV) velocity fields at four longitudinal planes across the width of the device. These fields showed there was substantial variation in the flow at the different planes, with a transfer of flow from the central planes during inflow towards the sidewalls during outflow, in addition to the outer planes spending a greater proportion of time in outflow and vice versa. This identified locations at which there is an even distribution between inflow and outflow. Divergence of the velocity fields was calculated to identify non-2D aspects to the flow revealing a vortex forming on the inner lip of the sidewall demonstrating the devices ability to utilise the volume outside of the extents of the sidewalls to generate power. This study has shown there are significant three-dimensional aspects to the flow within and around the device which must be considered when designing the underwater geometry.