Can i just take "hit" of oxygen when i need it?

No: A 'hit' of oxygen will do nothing for you if in fact your body needs oxygen. Once a condition has caused lung damage or heart damage to the point where supplemental oxygen is needed, then the oxygen is needed more or less all the time. Sometimes there is a temporary need for oxygen, such as with in pneumonia, but this requires oxygen until the infection clears.

Yes.: Isn't that what we all do? Well... Technically that is not exactly correct. Breathing is controlled by need to ventilate ( get rid of co2) and the need to oxygenate ( keep o2 levels up).