Significance As an essential nutrient, nitrogen plays a critical role in regulating oceanic primary productivity. Nitrogen is cycled between bioavailable and nonavailable forms through a network of aerobic and anaerobic microbial processes. Expanding oxygen minimum zones are hot spots for such transformations. Using a highly sensitive oxygen-sensing technique, we demonstrate that two key aerobic processes, ammonium oxidation and nitrite oxidation, persist even at low oxygen levels of 5–30 nM (∼0.01% air saturation). Activity at these low oxygen concentrations allows ammonium and nitrite oxidation to persist at oxic/anoxic boundaries and within anoxic waters if oxygen intrudes at sufficient frequency. In such environments, ammonium and nitrite oxidizers compete with anaerobic organisms for ammonium and nitrite, and thus act to control nitrogen loss.