Jim Martin wrote:

Today I took out a bunch of oak furniture that I built years ago for a
new coat of oil finish. Getting it out in the sunlight I was surprised
by how yellow it looks now (its about 9 years old).

Oil yellows. Always does, always will. Linseed goes bilious yellow in a
few months over a summer, better oils take some years to get there.
Only way to really stop it is to use a reddish oil instead. It still
adds some yellow, but at least it now looks an attractive brown, not
jaudiced..

Would I have any luck trying to fume it now?

Only if you can get the oil finish off first. You _might_ get some
ammonia through the oil in patches (especially worn edges), but any
colour change will be patchy as anything and not a good look.

I've successfully fumed oiled oak, but only on Craftsman-style stuff
where it was all square corners and easily sanded free of finish first.
Even then I had trouble with any sorts of moulding (but they were
already quite dark and the variation wasn't obvious)..

Also if there's any shellac around, ammonia has a bad effect on that.