We synthesize daily sonde (vertical) information and daily satellite (horizontal) information to provide an empirical description of ozone origins over the northern Indian Ocean during the INDOEX (Indian Ocean Experiment) field campaign (February–March 1999). This area is shown to be a significant portion of the “high-ozone tropics”. East-west O3 features and their flow are identified, and ozone origins are compared to other tropical regions, using water vapor as a second tracer. In the study period, multiple processes contribute to O3 column enhancements, their importance varying strongly by latitude: (1) Low-altitude O3 pollution over the northern Indian Ocean mainly originates from the Indian subcontinent and is traceable to high emission areas. Convective activity south of Sri Lanka helps direct ozone outflow from the northern Indian subcontinent. (2) Middle tropospheric O3 maxima over the northern Indian Ocean originate from various sources, often transitioning within a few hours. Convective venting of Asian pollutants can add 20–30 ppbv to the middle troposphere at 5°N–10°N, alternating with stratospheric influence. (3) A number of cases suggest that strong mixing-in of stratospheric air along the subtropical jet raised tropospheric O3 in early March by ∼40–50 ppbv, especially poleward of ∼10°N. (4) Influences of lightning and large-scale biomass burning were not strong during this period, in contrast to the situation in Africa and the South Atlantic or locally in Southeast Asia. This work illustrates successes and limitations in approaches to synthesizing disparate information on trace-gas distributions taken from satellite retrieval products and ozonesondes.