would it be possible for someone to be allergic to oxygen

No, they'd die.

well if they were they would probably die as a baby.

if you were from an all helium universe, yes. the oxygen would be far too dense and would probably drown the ''person''.

To be a little bit more specific, the way allergies work, essentially, is that your immune system becomes hypersensitive and begins attacking otherwise innocuous things, even though they serve no threat (and may actually be beneficial to) your body. Because of how much we rely on oxygen, we would simply be unable to live if our immune system was constantly regarding oxygen as a threat and reacting to it.

no, duh, y did u even waste ur time asking.

that one would be dead then and you can say he was allergic to oxygen ,