Technologies for medicinal oxygen delivery at home are increasingly part of the global health technology landscape in the face of rising rates of chronic lung and heart diseases. From the mere notion of harvesting and privatizing oxygen from the atmosphere to its status as both dangerous and therapeutic, and finally to its capacity to both extend and limit life, oxygen as therapy materializes its status as an ambivalent object in global health. This analysis of ethnographic material from Uruguay and South Africa on the experience of home oxygen therapy is guided by philosopher Don Ihde’s postphenomenology – a pragmatic philosophical approach for analysing the relationships between humans and technologies. Participants related to their oxygen devices as limiting-enablers, as markers of illness and measures of recovery, and as precious and limited resources. Oxygen was materialized in many forms, each with their own characteristics shaping the ‘amplification/reduction’ character of the relationship as well as the degree to which the devices became ‘transparent’ to their users. Ihde’s four types of human–technology relations – embodiment, hermeneutic, alterity and background relations – are at play in the multistability of oxygen. Importantly, the lack of technological ‘transparency’, in Ihde’s sense of the term, reflects not only the materiality of oxygen but inequality too. While postphenomenology adds a productive material and technological flavour to phenomenology, the author argues that a critical postphenomenology is needed to engage with the political-economy of human–oxygen technology relations.