I've received a few emails in the past several months inquiring about Oscar's condition post-surgery, so I figured I would post a short update here in response for anyone who is curious.
Oscar is doing very, very well and it seems his problem was the crystals in his bladder, rather than some sort of ongoing FLUTD (feline lower urinary tract disease), although the very nature of FLUTD is that it comes and goes indiscriminately so it's possible he still has that but it's calmed down because the crystals were removed. In either case, he's back to being a healthy, normal cat.
We've permanently switched both Oscar and Felix to crystal-preventing food so as to rule out any future crystal build-up, and the only side-effect seems to be thinner cats. Rather than being ten-point-five and eleven pounds respectively, they're each half a pound lighter, and a little thinner. It doesn't seem to be a problem, but they're due to visit the vet again soon so we'll know then. Neither seems any less energetic or, well, insane, so I don't think the weight loss has had a negative effect on them.
The only thing that has changed since Oscar returned to health is that he seems to be more aware of his own boredom. Whenever he wakes up from a nap or stops playing with a toy and realizes that he wants attention or interaction, he starts crying and whining like a baby until someone talks to him or picks him up or pets him. Oh, and he's grown a large affection for sitting in the bathroom (I think this came from when we placed a smaller, secondary litter box in there for him while he was sick).
Thanks very much to everyone who sent me nice emails about Oscar, I appreciate it.