Your breathing reflex is triggered by CO2 levels rising.
If you are breathing O2 through a mask you will not notice any diference (assuming you are healthy to begin with). If you are in a closed environment with O2 as you breath you convert O2 to CO2 and trigger your reflex (start to pant).
Elevated CO2 levels will result in your body running anarobicaly. Muscles will produce latic acid and you will feel pain, similar to cramp without the spasm. You will have dificulty in concentrating and in motor co-ordination. You will turn a bluish hue and eventually die.
If you receive O2 early enough you will recover but may have side effects such as headaches or memory loss, or even brain damage if you were starved of O2 long enough.
Elevated levels of O2 do not cause problems unless you are under pressure when you may suffer from Oxygen Narcosis. This will lead to death!
Marvelous stuff O2 .... too much and we die .... too little and we die.