Aston Villa completed the signing of Blackburn defender Stephen Warnock on Thursday night for an undisclosed fee - believed to be around £8million.
Warnock has put pen to paper on a four-year contract which will keep him at Villa Park until the summer of 2013.
The 27-year-old passed a medical and agreed personal terms this afternoon and could come into contention for Sunday's home Barclays Premier League game with Fulham.
O'Neill, who signed Newcastle defender Habib Beye earlier this month, is also set to add Manchester City skipper Richard Dunne to his squad.
Villa have reached agreement with City over the fee for 29-year-old Dunn.
Warnock underwent a medical and agreed personal terms at Villa earlier today ahead of making the move to midlands club.
A Villa statement reads: 'Aston Villa have completed the signing of Stephen Warnock on a four year deal.
New challenge: Warnock joins Martin O'Neill's side from Blackburn Rovers for an undisclosed fee believed to be in the region of £8million
Warnock is relishing the challenge of breaking into the Barclays League top four after making the move from Blackburn to Villa.
He told AVTV: 'The main aim is pushing for that top four spot. That's the ambition.
'The manager is buying some really talented players - young and ambitious - and I'm sure they will see that top four spot as a target and goal.
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