In 2009, four of 16 chambers in the Mutriku breakwater-integrated Oscillating Water Column (OWC) were badly damaged by storms, probably due to breaking wave loads, and slam within the chamber. To minimize exposure of future plant to such risks, it is necessary to characterise wave conditions under which such an installation could experience impact loads. This characterisation can be crucial to controling the power-take off resistance to increase the survability of the device during extreme weather. Large scale physical model tests in the Grosse Wellenkanal (GWK) included a video camera installed inside the chamber facing the rear chamber wall. Pressure sensors in the ceiling of the chamber were utilised to quantify the water loads. In-chamber impact pressures of up to 8 ρgH were recorded on the chamber ceiling, associated with the ‘sloshing’ observed. The “sloshing” phenomenon is not uncommon and should be considered in design processes.