What immediate action should be taken for major bleeding?

Applying a tourniquet is the most effective immediate action for controlling major bleeding, especially in situations where the bleeding is severe and life-threatening. The tourniquet works by constricting blood flow to a limb, effectively preventing blood loss and allowing the victim a greater chance of survival until more advanced medical care can be provided.

In cases of arterial bleeding or when direct pressure is inadequate, using a tourniquet can be the difference between life and death. It's important to place the tourniquet above the site of the injury—preferably two to four inches above the wound, and to ensure it is tightened sufficiently to stop the bleeding.

The other options, while potentially helpful in different contexts, do not address major bleeding as effectively. CPR is critical in cases of cardiac arrest but is not appropriate for managing bleeding. Maintaining body temperature is important for overall casualty care but does not directly address the immediate threat of significant blood loss. Administering oxygen can be beneficial in some scenarios, particularly when a casualty is in shock or has respiratory distress, but again, it does not control bleeding. Therefore, applying a tourniquet is the prioritized action in the presence of major bleeding.