The air we breath is roughly 1/5 oxygen. That should be a surprise, considering how we're always told that the human body breathes oxygen. That also leaves the question, "What about the other ~80%?". But that's a question for another day, today I'll be talking about why breathing pure oxygen, or 100% oxygen, would be really dangerous.
Breathing pure oxygen at low pressures, like in space, should cause no major problems since that's exactly what astronauts in the Gemini and Apollo programs did for almost two weeks straight.
At high pressures, anything above 4x the normal atmospheric pressure, oxygen poisoning could occur. That's also one of the reasons why it's very dangerous to dive deep.
Furthermore, based on results collected from tests done on guinea pigs, pure oxygen would cause fluids to somehow accumulate in your lungs. I'm not sure why that happens, but it does. It also lowers your lung, or more precisely, alveoli, productivity. As a result, you would need to breath more often.
Some other side effects include chest pains, mucus buildup, nausea, dizziness, blurred vision, and seizures.