Oxygen (O-2) is a prerequisite for cellular respiration in aerobic organisms but also elicits toxicity. To understand how animals cope with the ambivalent physiological nature of O-2, it is critical to elucidate the molecular mechanisms responsible for O-2 sensing. Here our systematic evaluation of transient receptor potential (TRP) cation channels using reactive disulfides with different redox potentials reveals the capability of TRPA1 to sense O-2. O-2 sensing is based upon disparate processes: whereas prolyl hydroxylases (PHDs) exert O-2-dependent inhibition on TRPA1 activity in normoxia, direct O-2 action overrides the inhibition via the prominent sensitivity of TRPA1 to cysteine-mediated oxidation in hyperoxia. Unexpectedly, TRPA1 is activated through relief from the same PHD-mediated inhibition in hypoxia. In mice, disruption of the Trpa1 gene abolishes hyperoxia- and hypoxia-induced cationic currents in vagal and sensory neurons and thereby impedes enhancement of in vivo vagal discharges induced by hyperoxia and hypoxia. The results suggest a new O-2-sensing mechanism mediated by TRPA1.