XP 285 100% Oxygen

Breathing pure oxygen at low pressures, like in s.p.a.ce, 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 acc.u.mulate 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.