Climate change reduces ocean oxygen levels, posing a serious threat to marine ecosystems and their benefits to society. State-of-the-art Earth System Models (ESMs) project an intensification of global oxygen loss in the future, but poorly constrain its patterns and magnitude, with contradictory oxygen gain or loss projected in tropical oceans. We introduce an oxygen water mass framework– grouping waters with similar oxygen concentrations from lowest to highest levels– and separate oxygen changes into two components: the transformation of oxygen in water masses by biological, chemical or physical processes along their pathways in ’ventilation-space’, and the redistribution of these water masses in ’geographic-space’. The redistribution of water masses explains the large projection uncertainties in the tropics. ESMs with more realistic representations of water masses provide tighter constraints on future redistribution than less skilled ESMs, leading to over a third more of tropical area exhibiting consistent oxygen projections (58% vs 22%), and a 30% reduction in model spread for tropical oxygen projections. These higher-skilled ESMs also project weaker global deoxygenation than less skilled models (median of -6.5 vs -9.5 Pmol O2 per °C of surface warming) controlled by an increase in global water residence times, and they project a stronger increase in oxygen minimum zone ventilation by ocean mixing. These tighter constraints on future oxygen changes are critical to anticipate and mitigate impacts for ecosystems, and inform management and conservation strategies of marine resources.