Marine mammals rely on maintaining sufficient blood oxygen levels while diving to prevent drowning. Generally, oxygen is cognitively imperceptible to mammals that instead sense rising carbon dioxide as a proxy for low oxygen. Not perceiving oxygen, however, is risky for diving mammals. We argue that any ability to alter dives based upon direct perception of oxygen should have been strongly selected for. We exposed diving seals to inhaled gas mixes that were experimentally altered to affect circulating levels of oxygen and carbon dioxide. Dive duration was positively correlated with circulating oxygen levels but unaffected by carbon dioxide levels and pH. These results suggest that seals do cognitively perceive circulating oxygen and use this to alter dive behavior.