The capacity of airborne particulate matter to generate reactive oxygen species (ROS) has been correlated with the generation of oxidative stress both in-vitro and in-vivo. The cellular damage from oxidative stress, and by implication with ROS, is associated with several common diseases such as asthma, chronic obstructive pulmonary disease (COPD), and some neurological diseases. Yet currently available chemical and in-vitro assays to determine the oxidative capacity of ambient particles require large samples, analysis are typically done offline, and the results are not immediate. In this manuscript we report the development of an on-line monitor of the oxidative capacity of aerosols (o-MOCA) to provide on-line, time-resolved assessment of the capacity of airborne particles to generate ROS. Our approach combines the Liquid Spot Sampler (LSS), which collects particles directly into small volumes of liquid, and a chemical module optimized for on-line measurement of the oxidative capacity of aerosol using the dithiothreitol (DTT) assay. The LSS uses a three-stage, laminar-flow water condensation approach to enable the collection of particles as small as 5 nm into liquid, without subjecting the sample to temperature extremes. The DTT assay has been improved to allow the on-line, time-resolved analysis of samples collected with the LSS. The o-MOCA was optimized and its performance evaluated using the 9,10-Phenanthraquinone (PQ) as standard redox-active compound. Laboratory testing shows minimum interferences or carry-over between consecutive samples, low blanks, and a reproducible, linear response between the DTT consumption rate (nmol/min) and PQ concentration (µM). The calculated limit of detection for o-MOCA was 0.15 nmol/min. The system was validated with a Diesel Exhaust Particle (DEP) extract, previously characterized and used for the development, improvement, and validation of the standard DTT analysis. The DTT consumption rates (nmol/min) obtained with the o-MOCA were within experimental uncertainties of those previously reported for these DEP samples. In ambient air testing, the fully automated o-MOCA was run unattended for 3 days with 3-h time resolution, and showed a diurnal and daily variability in the measured consumption rates (nmol/min/m3).