Question: What is the ruling on using Oxygen which is placed on the mouth of the sick during most of the day. Does this break the fast?
Answer: ” Oxygen which is given to the sick does not break the fast because this Oxygen is not a mass that reaches the stomach such that we can say: it is food and drink. However, it is cold and opens the arteries so a person can breath. So based upon that it is not food or drink. What is not food and drink and is not under the meaning of food and drink, then it does not break the fast”.
[Taken from: http://shamela.ws/browse.php/book-7679#page-305]
Faisal Ibn Abdul Qaadir Ibn Hassan