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 on Thursday afternoon and could come into contention for Sunday's home 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.
A Villa statement reads: "Aston Villa have completed the signing of Stephen Warnock on a four year deal.
"We can also confirm that a fee has been agreed with Manchester City for defender Richard Dunne. The player will now discuss terms on a move to Villa Park."
Warnock has made 87 appearances in two years at Ewood Park since moving from Liverpool.
He earned his one England cap to date as a substitute against Trinidad and Tobago during the summer of 2008.
O'Neill has been searching relentlessly to strengthen his back four after inspirational captain Martin Laursen was forced to retire through injury.
His numbers were further reduced after he agreed to off-load central defender Zat Knight to Bolton.
Villa also agreed a fee with Portsmouth for Sylvain Distin but he is likely to move to Everton as a replacement for Joleon Lescott.
Dunne is not the paciest of defenders but a solid sort of player who will compliment the club's only other two centre-backs Carlos Cuellar and Curtis Davies who is dogged by a shoulder problem.
O'Neill has so far spent only a net £6million after buying Stewart Downing, Fabian Delph and Habib Beye for £21million and off-loading Gareth Barry and Knight for a combined £15 million.
But the midlands club will splash out up to £8million for Warnock plus £6million for Dunne.
Dunne has spent the last nine years at City, clocking up close to 300 appearances, but the arrival of Kolo Toure and Lescott means his days look numbered as a first choice player.
He is a whole-hearted and committed player who has been sent off eight times in the Premier League, equalling the record number of dismissals held by Patrick Vieira and Duncan Ferguson.
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.
"Playing in Europe is important - playing against the top players on the continent is fantastic - and there's no better stage to do that than on the Champions League.
"If we could do that, it would be a great achievement."