From Wikipedia;
It has been claimed by alternative medicine that the human body is oxygen deprived, and so that more oxygen will remove "toxins" and even cure cancer. Proponents claim this practice is not only safe, but enhances health and well-being, including strengthening the immune system, enhancing concentration, reducing stress, increasing energy and alertness, lessening the effects of hangovers, headaches, and sinus problems, and generally relaxing the body. It has also been alleged to help with altitude sickness. However, no long-term, well-controlled scientific studies have confirmed any of the proponents' claims. Furthermore, the human body is adapted to 21 percent oxygen, and the blood exiting the lungs already has about 97 percent of the oxygen that it could carry bound to hemoglobin. Having a higher oxygen fraction in the lungs serves no purpose, and may actually be detrimental.
The medical profession warns that individuals with respiratory diseases such as asthma and emphysema should not inhale too much oxygen. Higher than normal oxygen partial pressure can also indirectly cause carbon dioxidenarcosis in patients with chronic obstructive pulmonary disease (COPD).