The oxygen that is given to some patients does not invalidate the fast, because nothing else is added to this oxygen, so it comes under the same ruling as breathing natural air.
Hence in a statement of the Islamic Fiqh Council it says:
The following things are not regarded as things that break the fast: … oxygen. End quote.
See: Majallat Majma’ al-Fiqh al-Islami (10/2/96, 454), Muftiraat al-Siyaam al-Mu’aasir, by Dr. Ahmad Khaleel, p. 50.