I'm english so it's probably slightly different for me, but one of the veterinary hospitals i did work experience in had special kennels which could be used as oxygen tents. I didn't see them used but they were there.
As for the other question, i have no idea i'm afraid!
Oxygen is administered during general anesthesia during all but the most minor procedures.
The intensive care unit in veterinary teaching hospitals all have oxygen that can be delivered in several ways, typically by mask, nasal catheter, oxygen cage, or ventilator.
Almost all private veterinary hospitals have oxygen, in fact it's required by law in many states. However they may or may not have an oxygen cage as these can be fairly expensive.
Also, (and I'm not sure where you were going with this) you can't really strap an oxygen tube and a little roller O2 tank to an animal and have them just run around with it...so generally conditions where one would do this in human med are or soon will be an end-of life situation for an animal. If there is a temporary condition, you can put them in the O2 cages, but again if they're going to need O2 24-7 there aren't that many options.