We breathe air that is 21 percent oxygen, and we require oxygen to live. So you might think that breathing 100 percent oxygen would be good for us -- but actually it can be harmful. So, the short answer is, pure oxygen is generally bad, and sometimes toxic. To understand why, you need to go into some detail …
Your lungs are basically a long series of tubes that branch out from your nose and mouth (from trachea to bronchi to bronchioles) and end in little thin-walled air sacs called alveoli. Think of soap bubbles on the end of a straw, and you'll understand alveoli. Surrounding each alveolus are small, thin-walled blood vessels, called pulmonary capillaries. Between the capillaries and the alveolus is a thin wall (about 0.5 microns thick) through which various gases (oxygen, carbon dioxide, and nitrogen) pass.
When you inhale, the alveoli fill with this air. Because the oxygen concentration is high in the alveoli and low in the blood entering the pulmonary capillaries, oxygen diffuses from the air into the blood. Likewise, because the concentration of carbon dioxide is higher in the blood that's entering the capillaries than it is in the alveolar air, carbon dioxide passes from the blood to the alveoli. The nitrogen concentration in the blood and the alveolar air is about the same. The gases exchange across the alveolar wall and the air inside the alveoli becomes depleted of oxygen and rich in carbon dioxide. When you exhale, you breathe out this carbon dioxide enriched, oxygen-poor air.