Dissolved oxygen is essential for marine ecosystems and biogeochemical cycles, and its ongoing decline in a warming ocean threatens ecosystem health and fisheries. The ocean's oxygen inventory reflects a balance between physical air–sea supply and interior consumption, where preformed oxygen captures the physical component of oxygen injected at the surface and transported into the interior. While much attention has focused on how solubility and air–sea disequilibrium regulate preformed oxygen under warming, the role of interior circulation in redistributing this oxygen signal remains poorly quantified. Here we conduct an idealized warming experiment with a global MITgcm configuration and introduce a suite of passive tracers that isolate the surface oxygen imprint, interior physical effects, and water-mass source regions. In the upper 100–1000 m, preformed oxygen increases in parts of the subtropical thermocline and upper-intermediate ocean even as surface oxygen saturation declines. By prescribing a fixed surface imprint, we show that these positive anomalies arise entirely from changes in interior transport and water-mass structure rather than from an enhanced surface oxygen boundary condition. A decomposition into along-isopycnal property changes and isopycnal heave reveals that lateral redistribution along neutral surfaces raises preformed oxygen across wide low- and mid-latitude regions, whereas vertical heave often offsets this gain at fixed depth levels. Source-region diagnostics link the positive anomalies to more poleward water-mass origins and an increased contribution from high-latitude surface waters, indicating that ventilation pathways shift toward better-ventilated sources under warming. These results demonstrate that circulation-driven reorganization of preformed oxygen can partially counteract upper-ocean oxygen loss, highlighting a dynamic buffering mechanism that cannot be captured by thermodynamic changes alone. The tracer-based framework developed here provides a systematic approach for attributing oxygen variability to specific source regions, offering new insight into how changing water-mass formation and transport pathways shape the ocean’s oxygen response to climate change.