There are two components to hyperbaric oxygen: increased ambient pressure and increased inspired oxygen concentration.
In essence, the amount of a gas (such as oxygen) that is dissolved in a liquid (such as blood) is directly proportional to the concentration of the gas at the surface of the liquid and the ambient pressure that the whole system is under.
By having a patient breathe 100 percent oxygen while in a pressurized chamber, hyperbaric oxygen therapy increases the amount of oxygen in the patient's blood. This is called hyperoxia.
HBO therapy has other effects, such as shrinking air bubbles in the blood or tissue (as occurs in decompression illness—also known as the bends) and lessening the effects of inflammation and re-perfusion injury. The three factors above are the key to therapy for most diseases treated by HBO.