O'Neill is an admirer of West Ham centre-back Matthew Upson but is put off making a formal bid for the 30-year-old by an asking price of around £12m.
Portsmouth defender Distin would cost around £5m with Villa already having had two offers rejected for the former Manchester City player.
O'Neill is reluctant to pay over the odds for 31-year-old Distin, who is the Pompey captain, and may play a waiting game right up until the end of the transfer window.
The former Celtic boss is also in the hunt for West Ham's Collins who has also been a target for Stoke. O'Neill is lining up a bid of around £5m for the Welsh international.
He is desperate to bring in proven quality at the heart of his back four to supplement Carlos Cuellar and Curtis Davies.
The retirement of Martin Laursen and sale of Zat Knight to Bolton for £3m has left him short in that department.
Warnock has also been tracked for several months with Villa already having had one bid turned down for the Blackburn full-back.
He is keeping his fingers crossed that Rovers boss Sam Allardyce may make Warnock available after signing Real Madrid defender Michel Salgado.