I did this recently, had to for a rat. It didn't end well for me but here goes on what I learned:
You have to be *very careful* about how muich oxygen you give, you can't just give pure oxygen, there needs to be outside ventalation too, enough to keep the oxygen concentration up, but not too much. If you're seeing frequent yawning, it's too high, if the animals feet and tail are not the appropraite pink color but rather purple/blueish or purple, or red, you've got it too low(or way, way too high) and once you put an animal ON oxygen, it takes a lot of time a LOT of time to safely wean them off of it!!!! You must take many hours to wean an animal slowly off of oxygen or terrible things will happen, seriously! That part is not easy or fun.
I used a 1-2 liter I believe plastic shoebox type storage thing with attached lid(sterilite) I got at walmart, I drilled a hole for the oxygen cord adn propped the lid open a little bit for ventilation and it seemed to work well, but it was only big enough for one female to stretch out in one food bowl and a blankie on the bottom.
If you used a say 10 gallon tank, with holes drilled in a piece of plastic or plexi-glass for the top, be sure of enough outside ventilation, that would work better for long-term I would think. More room to move around. eat, be comfy. The larger the space you need to fill with oxygen, the more you will need(bigger tank) obviously.
EDIT: It was expensive, I just went to my local human medical supply place, my vet called in the script, I rented a refillable tank for $50. The tank was too small, you can get them that would last 2-3 days or at least 24 hours depending on waht setting, but you'll have to pay to have it refilled. I believe each refill was 12$ for my small tank. you'll have to work out a carrier that is oxygenated to take the rat to the vet, and somehow something while you're switching tanks.
EDIT AGAIN: something like lasix would help get teh fluid out, but you've got to be attacking the infection hard core at the same time and work real hard to keep the rats hydration up.