Ocean oxygen contents are predicted to fall as the Earth warms. Here, we use sediment records from the warm Pliocene, when atmospheric CO2 concentration, ocean circulation, and average temperatures of 3-5° above present day, approximate predicted future conditions, to evaluate the extent of suboxia (O2< 10µM) in the ocean during a warm climate. New high-resolution nitrogen isotope records from the eastern equatorial Pacific, together with existing records from the North Pacific and the Arabian Sea and a broader multi-site survey, show intensification of oxygen minimum zones (OMZs) in the Indo-Pacific and the expansion of water column denitrification accompanying the cooling ~2.0 Ma. This may be due to the inception of a modern polar frontal system and mode water circulation; mode waters are the primary conduit of nutrients and oxygen to the low latitude thermocline and ultimately to the OMZ. These results run counter to the anticipated expansion of the OMZs with warming and stress how climate-related changes in circulation may complicate our ability to predict future changes.