Pacific Shadow Zone (PSZ) ventilation relies primarily on diffusive exchange rather than large-scale overturning, yet the mechanisms governing its oxygenation since the last glacial maximum (LGM) remain poorly constrained. Here we present a high-resolution quantitative reconstruction of bottom-water oxygen concentrations from sediment core KT-1 (1,329 m) in the northern South China Sea, ventilated by Pacific Deep Water originating within the PSZ, providing a downstream record of PSZ ventilation and also constraining the lower limit of North Pacific Intermediate Water (NPIW) influence. Bottom-water oxygen remained persistently low throughout the LGM and Heinrich Stadial 1, indicating that the lower limit of deglacial NPIW influence remained above 1.3 km, before increasing rapidly during the Bølling–Allerød and continuing into the early Holocene. The oxygen evolution was decoupled from local productivity changes, highlighting the dominant control of sourced-water ventilation. Integrating proxy compilations with transient climate-model simulations, we show that PSZ oxygenation was jointly controlled by the oxygen content of Southern Ocean source waters and the efficiency of intermediate–deep exchange modulated by North Pacific stratification, both linked to Atlantic Meridional Overturning Circulation variability. These findings identify the PSZ as a dynamically regulated component of the overturning system, with important implications for deglacial carbon storage.