This retrospective cohort study of 343 reoperative cardiac surgery patients found that low indexed oxygen delivery (DO₂i) during cardiopulmonary bypass independently predicted in-hospital mortality and major morbidity. A median DO₂i below 289 mL/min/m² was associated with a fourfold increase in mortality risk, higher rates of acute kidney injury, cardiac complications, and prolonged ventilation. Optimizing intraoperative oxygen delivery may improve outcomes in this high-risk population.