Understanding the oxidizing environment indoors is important for predicting indoor air quality and its impact on human health. We made continuous time-resolved measurements (30 s) of several oxidants and oxidant precursors (collectively referred to as oxidant*): ozone (O3), nitric oxide (NO), and NO2* – the sum of nitrogen dioxide (NO2) and nitrous acid (HONO). These species were measured in three indoor environments – an occupied residence, a chemistry laboratory, and an academic office – in Syracuse, New York, during two seasons in 2017 and 2018. Oxidant* levels differed greatly between the residence, the lab and the office. Indoor-to-outdoor ratios (I/O) of O3 were 0.03 and 0.67 in the residence and office; I/ONO (I/ONO2*) were 11.70 (1.26) in the residence and 0.13 (1.70) in the office. Little seasonal variability was observed in the lab and office, but O3and NO2* levels in the residence were greater in spring than in winter, while NO levels were lower. Human activities such as cooking and opening patio doors resulted in large changes in oxidant* mixing ratios in the residence. In situ chamber experiments demonstrated that the increase in O3 and NO2* levels during door-open periods was due to a combination of physical mixing between indoor and outdoor air, gas-phase production of NO2 from O3–NO chemistry, and heterogeneous formation of HONO on indoor surfaces. Our results also highlight the importance of chemistry (with NO, alkenes, and surfaces) in O3 mixing ratios in the residence, especially during door-open periods.