Strange kitty behavior toward sick kitty ...?

It's pretty common for the cat(s) that remain home to be hostile to a cat who has gone to the veterinarian. Because cats identify members of their pride mostly by scent (that's the point of the whole nose-to-nose greeting, as well when kittens present their backsides to their mommas to be sniffed, which some adult cats continue to do to their humans), the scents that are picked up in the course of a visit to a veterinarian do make it hard for the other cats to recognize Oscar. Don't worry, as he grooms himself and hangs around at home he will start to smell like himself again.