I read that the chambers were repurosed near the end of the war to be air raid shelters, how was oxygen supplied then to the chambers?
The Birkenau gas chambers were not repurposed and have never had anything to do with air-raid shelters.
The above-ground gas chambers in the crematoria 4 and 5 were relatively flimsy and thus useless for this purpose anyway.
Same with the Bunker gas chambers converted from peasant houses.
The much more massive gassing cellars of the crematoria 2 and 3 would have done the job, especially as they had adequate supply/exhaust ventilation systems, but obviously both their original intended function (morgues) and their actual function (homicidal gas chambers) was incompatible with being used as an air-raid shelter and there's not a snippet of documentary evidence for such a "repurposing" anyway.
The only gas chamber that was turned into an air-raid shelter was the small provisional gas chamber in the crematorium 1 in the main camp (the one shown to the tourists).
It happened in 1944, when the crematorium of the main camp was no longer in use and the mass extermination and incineration was taking place in Birkenau.
Pressac notes in regard to the converted Krema:
"It is not known what type of ventilation fans, if any, were installed."
The conversion involved not only sealing various openings (incl. Zyklon B introduction holes) but also creating numerous small rooms in the former morgue/gas chamber.
After the liberation by the Soviets it was attempted to bring the building into its original state: the new walls were removed, the openings in the ceiling reopened, the ovens partially reconstructed. They even rebuilt the demolished chimney (as a "dummy" of course).
Unfortunately the attempted reconstruction was partially a failure - e.g. they removed one wall too many and the current room is now larger than the original gas chamber since a former washroom is how a part of it (incl. visible sewer pipe). Holocaust deniers don't get tired of misusing this mistake.
Sources: Pressac, Auschwitz: Technique and Operation of the Gas Chambers; Piper, Pressac and van Pelt in the Anatomy of the Auschwitz Death Camp; the 5-vol. history of Auschwitz by the Auschwitz Museum; and numerous Auschwitz Zentralbauleitung documents.