The major side effects seem to be: Pee a lot, drink a lot, eat a lot. I have heard of personality changes (aggression), but not seen it. Minnie was once on such a high dose that she started "shedding". Here's some stuff on pred at PetPlace.com (see Precautions and Side Effects).
I'm not fond of the side effects, but pred has saved two of my girls (one canine, one feline) and I'll use it again if need be.
ETA: I sure hope it works for Oscar.