The development of oscillating water column (OWC) wave energy converter (WEC) has been very extensive in the last couple of decades with the successful grid connection of the European pilot projects such as, LIMPET, OWC Pico power plant, and Mutriku wave energy plant. The water column behaviour in the OWC chamber, however, usually assumed to be well behaved during operation in the design process. This assumption later proven to be not accurate with damaged received inside the caisson chamber, most likely due to chamber sloshing. This paper aims to observe the water column behaviour during various type of wave condition for both regular and irregular wave settings. The experiment involves a small-scale physical model, pressure transducer, and wave gauges in the University of Edinburgh long wave flume facility. The results show that sloshing is more likely to occur in the longer wave length relative to the chamber width, higher wave height, and less chamber pressure generated. Four types of water column behaviours (well behaved/no sloshing, low sloshing, medium sloshing, and high sloshing) was characterised. A ceiling impacts up to at least 1.25ρgH was observed during sloshing.