It may be columnaris - I didn't want to freak you out. If it is, your fish should be dead in a week, not weeks.
Please take the oscar out into the 10 G whether it's cycled or not does not matter. Change the water out of the 10G 50% a day to keep the ammonia down. It will be fine.
Increase the water changes in the angel tank to every other day. Vacuum the substrate well, rinse out your filter media with a bucket of tank water, wipe down the sides of your filter.
I agree with Oo, that lowering the temps to to 77 or even 76, if angels can take that temp.
I would never recommend using antibiotic in the display tank; however, it's your choice. You could pick up a 20 gallon, put all your angels in there with a sponge filter, heater, and airstone; change out water 50% a day, redose antibiotics. Treat for 10 days to ensure that the bacteria is killed. Some recommend only 7 days, but IMO, you could be letting some survive and thereby creating a new super bacteria.
There is one other option, but it's risky; I prefer it over antibiotics for columnaris, fungus and other surface infections and parasites. It's a potassium permanganate bath.