Ocean oxygen, which is critical for marine life, is currently declining as oceans warm and circulation slows down. Despite this, the future of marine oxygen under a warming climate is poorly understood. Modeling past warm climates such as the Miocene Climatic Optimum (16.9 to 14.7 million years ago) offer us the opportunity to investigate the processes that have influenced ocean oxygen during warm periods in Earth's history. Despite modern expansion of deoxygenated zones in the ocean, proxy evidence suggests that deoxygenated zones contracted in the Eastern Pacific during this warm episode. We present new biogeochemical model simulations that allow us to reconcile these proxy trends. We find that, in some simulations, deoxygenated zones contract under higher levels of CO₂ due to reduced remineralization. This occurs as a result of reduced wind-driven upwelling of deep, nutrient rich water, which is linked to a slower atmospheric circulation at higher CO₂ levels.