any compound of oxygen with another element or a radical

a highly poisonous brown gas (NO2)

a heavy odorless colorless gas formed during respiration and by the decomposition of organic substances; absorbed from the air by plants in photosynthesis

an explosive gas (ClO2) used chiefly in bleaching paper or starch or soap or flour and in water purification

a colorless toxic gas (SO2) that occurs in the gases from volcanoes; used in many manufacturing processes and present in industrial emissions; causes acid rain