This study introduces a new online system for measuring oxidative potential (OP), using the dithiothreitol (DTT) assay with a direct-to-reagent approach. The instrument enables real-time OP^DTT assessments with 5-minute resolution and a competitive instrumental detection limit (0.066 nmol_DTT min^-1). Its performance was evaluated using model compounds and under controlled experiments in the CESAM atmospheric simulation chamber. The system exhibited clear and consistent responses to major OP-active species, including quinones, copper, manganese, secondary organic aerosols (SOA), and black carbon. In chamber experiments conducted without photochemistry, the OP_v^DTT values remained consistently lower than during experiments involving photochemistry, despite the higher concentrations of metals and similar particulate masses. This finding underscores the critical role of photochemically produced species in driving OP. Overall, the instrument demonstrated its capacity to monitor OP_v^DTT at a high temporal resolution, paving the way for a deeper understanding of the influences on OP from emission sources and atmospheric processes, particularly transient ones.