Who or whom?

[ChipInTheSugar]

"He was one of the few people on who/whom she could rely."

I'm leaning towards who, but I'm not 100% certain - can any experts please tell me which it should be?

Thank you

Whom. Definitely.

Whom is the object of a preposition, so "on whom".

Rule of thumb is that if the pronoun has a preposition it's "whom".
Nobody (not even English teachers) will red pen "who" though.
MN pedants will get a bit shrieky, mind.

I try to correlate 'whom' with 'him'. If the answer makes sense with 'him' I use 'whom'. On whom did she rely? She relied on him.

(I think “upon whom” sounds better).

Whom

If you are writing a literary novel:
one of the few people on whom she could rely

if you're chatting down the pub and don't want to appear pompous:
one of the few people who she could rely on
(or even drop the who altogether)