The global ocean overall is losing oxygen under climate warming, yet most climate models project increasing subsurface (100–1,000 m) oxygen concentrations in the tropical "shadow zones" — poorly ventilated low-latitude regions of the Indian, eastern Atlantic, and eastern Pacific Oceans. Previous studies have attributed this projected oxygen increase partly to improved ventilation in the tropical subsurface, often explained by enhanced tropical stratification that suppresses upwelling. However, such local stratification changes cannot readily account for the basin-scale reduction in tropical upwelling projected under climate warming. Here we show that the improved tropical subsurface ventilation arises primarily from remote extratropical forcing. Using climate model simulations and a suite of process-based ocean experiments, we demonstrate that extratropical warming and freshening weaken subtropical water mass formation and induce cross-hemispheric overturning adjustments that deepen isopycnal surfaces across ocean basins. This adiabatic deepening displaces well-ventilated, oxygen-rich shallow waters to greater depths, thereby improving ventilation and oxygenation of low-latitude subsurface waters. These results provide a dynamical explanation for the projected oxygenation response in tropical shadow zones and highlight the central role of ocean circulation and extratropical processes in shaping future ocean oxygen distributions.