Investigations of enzymatic reactions in supercritical CO, are often hindered by the high pressure involved in these processes, making reaction monitoring extremely difficult. This paper describes the implementation of a fiber optic based oxygen sensor into a high pressure reactor for supercritical carbon dioxide. The sensor is pressure resistant, working in supercritical carbon dioxide and reusable after depressurisation. The sensor signal is found to be affected by pressure changes, but stable at constant pressure. Oxygen concentration in supercritical CO2 is monitored using the disproportionation of hydrogen peroxide as a simple oxygen producing reaction.