There are potential dangers of routine use of supplemental oxygen in patients with medical emergencies. High-dose oxygen is associated with a variety of hemodynamic alterations that may increase myocardial ischemia and impair cardiac performance. This viewpoint outlines that supplemental oxygen therapy may also be associated with adverse outcomes in several non-cardiac emergencies. The literature to date is sufficiently convincing to recommend appropriate caution in applying supplemental oxygen. Severe hypoxemia should be treated promptly but slowly with stepwise increases in oxygen concentration, avoiding arterial hyperoxemia. It appears reasonable to aim at a peripheral oxygen saturation of 90% to 94%, particularly if the clinical condition of the patient is stable at that point. More precise titration of oxygen is obviously facilitated by arterial blood gas analysis.