Oxygen (O2) supplementation increases the O2 content of blood, increases the partial pressure of oxygen (PO2) in the capillary blood, and improves tissue delivery of O2. In addition to improving tissue oxygenation, the administration of O2 may improve the function of O2-dependent cellular systems, such as the cytochrome P450 system, which is important to drug and toxin metabolism; nitric oxide synthase, which regulates vasodilation; and host defense systems. Improved tissue oxygenation is also beneficial for wound healing. Given the important contributions that supplemental O2 can make, it is no wonder that O2 is one of the most common drugs administered in the emergency and intensive care settings.