Fixation of carbon dioxide (CO2) through photosynthesis, which drives the global carbon cycle, is catalysed by the most abundant enzyme on Earth, ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCo). Despite its evolutionary success, a major problem of this enzyme is its propensity to fix molecular oxygen (O2), which is a competitive inhibitor of CO2. Each year, ~100 gigatons of oxygen are fixed by RuBisCo, causing a huge liberation of CO2 through photorespiratory metabolism. Surprisingly, our understanding of how O2 reacts with RuBisCo has remained mysterious for decades.