Ugh, sorry about this whole debacle, Katie.
Main thing will be to get his temporary 20 gal. home in order first. All you really need to begin with is a heater and some source of surface agitation (a seeded filter being much better, of course, along with seeded gravel).
I'm assuming you've at least gotten that done already, so Meaty should do fine for a few days while you get a more permanent fix for him. I'd do daily water changes to keep nitrAtes down, and I'd strongly recommend minimizing feedings as much as possible. As much as I hate to suggest depriving a growing oscar of food, a few days shouldn't hurt, and it'll go a long way toward helping with water quality while he's waiting on his permanent home to be ready for him.
Hang in there!
BV