In some very special cases, SHORT TERM exposure to higher levels of oxygen can do more good than harm. Some burn victims gain more in terms of healing than they lose in terms of their inner body. However, there have been experiments which clearly show that long exposure to high levels of oxygen is bad. When guinea pigs were exposed to pure oxygen at normal atmospheric pressure for 2 days, fluid accumulated in the lungs, and the epithelial cells lining the alveolus and pulmonary capillaries were damaged. 

Human beings (intensive-care patients on breathing machines at 30
hours or more exposure or volunteers with 24 hour exposure) who had
long-term exposure to pure oxygen at normal atmospheric pressure suffered
the following problems:

1. The same fluid buildup and tissue damage as the guinea pigs.

2. Decreases in the rate of gas exchange across the alveoli (probably due
to the tissue damage)

3. Chest pains that were worse during deep breathing

4. Decrease in the total volume of exchangeable air in the lung

5. Local areas of collapsed alveoli

6. Blindness caused by inadequate development of the capillaries in the
lens and retina of the eye (this happened in premature infants who had to
be on high oxygen content because of underdeveloped lungs). These data
made physicians decide that even premature infants cannot be exposed to
radically high levels of oxygen, even though their breathing is poor.
Unless a premature baby is in danger of suffocation due to inadequate
lungs, they are exposed to lowest level of oxygen (near 21%) possible.