Nanostructured materials characterized by high surface–volume ratio hold the promise to constitute the active materials for next‐generation sensors. Solution‐processed hybrid organohalide perovskites, which have been extensively used in the last few years for optoelectronic applications, are characterized by a self‐assembled nanostructured morphology, which makes them an ideal candidate for gas sensing. Hitherto, detailed studies of the dependence of their electrical characteristics on the environmental atmosphere have not been performed, and even the effect of a ubiquitous gas such as O2 has been widely overlooked. Here, the electrical response of organohalide perovskites to oxygen is studied. Surprisingly, a colossal increase (3000‐fold) in the resistance of perovskite‐based lateral devices is found when measured in a full oxygen atmosphere, which is ascribed to a trap healing mechanism originating …