Aston Villa have completed a £6.5m deal for Celtic midfielder Stilian Petrov.
Petrov is O'Neill's first signing
The 27-year-old Bulgaria captain has signed a four-year contract and is boss Martin O'Neill's first signing since taking over at Villa Park.
Petrov worked with O'Neill at Celtic and it is understood the transfer could eventually be worth about £8m.
"I've known him for quite some time and I think he is athletic, young and can also score some goals for us," O'Neill told Villa's official website.
"I think his best years are ahead of him, at 27. He's the right age but he's also got plenty of experience behind him."
Petrov is away on international duty, but loose ends have been tied up and is likely to make his Villa debut against West Ham on 10 September.
He's got that winning mentality from his years at Celtic
Aston Villa boss Martin O'Neill
Portsmouth boss Harry Redknapp had trailed Petrov throughout the summer, but admitted defeat once O'Neill made his move.
The talented Petrov joined Celtic in July 1999 from CSKA Sofia and has made more than 250 appearances for the club.
Petrov was contracted to the Scottish champions until 2010 but submitted a transfer request at the end of last season.
He won his first cap for his country in March 1999 against Belgium and was eventually appointed skipper of his country.
Petrov was voted the Scottish Premier League's player of the year in 2001 after helping Celtic to the domestic treble.
"He's got that winning mentality from his years at Celtic but I think he had that anyway," added O'Neill.
"I am delighted that we've been able to sign him."
O'Neill is also believed to be interested in Barcelona striker Javier Saviola who is out of favour at the Nou Camp.
The 24-year-old is out of contract next summer and Barcelona are keen to off-load the Argentinian international who impressed at the World Cup.
But O'Neill faces competition from the likes of Celta Vigo, Real Zaragoza and Villarreal for Saviola.