One of the leading causes of the impact magnitude of COVID-19 in Peru was the lack of medical oxygen. What explains this oxygen shortage, which is high even by regional standards? This article presents a response from the perspective of the legacies of the Peruvian health system. First, a structural cause. The historical deficiencies of the health system and specific characteristics linked to outsourcing services led to a low capacity to provide this resource. Second, an institutional legacy. A 2010 regulation established a purity requirement of 99% for medical oxygen. This percentage, which is well above international standards, reduced the offer, concentrating it on two providers. The case of the Ayacucho region, which has a better supply of oxygen thanks to two high-volume tanks, illustrates some of these dynamics and the importance of this resource in saving lives. One of the leading causes of the impact magnitude of COVID-19 in Peru was the lack of medical oxygen. What explains this oxygen shortage, which is high even by regional standards? This article presents a response from the perspective of the legacies of the Peruvian health system. First, a structural cause. The historical deficiencies of the health system and specific characteristics linked to outsourcing services led to a low capacity to provide this resource. Second, an institutional legacy. A 2010 regulation established a purity requirement of 99% for medical oxygen. This percentage, which is well above international standards, reduced the offer, concentrating it on two providers. The case of the Ayacucho region, which has a better supply of oxygen thanks to two high-volume tanks, illustrates some of these dynamics and the importance of this resource in saving lives.