Greg,
How old is Oscar? You mentioned he "chews" after he's been fed -- is he
grinding his teeth? It sounds to me like he has gastric ulcers. I had a
ferret who used to do the same thing: one day she'd be fine and the next not
eating. I knew she had ulcers because she would just grind her teeth like
crazy.
I'd visit the vet with this possibility and see what he/she says.
Amy & Dave (and their gaggle of giggling ferrets)
RIP Ian and Elektra
[Posted in FML issue 1628]