The Villa boss left Glasgow in 2005 to care for his sick wife before taking the job at Villa Park a year later.
His decision brought to an end a highly successful reign at Celtic Park, where he took the club to three league titles, three Scottish cups and the UEFA Cup final in 2003, in which they lost to Porto in extra time.
O'Neill has enhanced his reputation at Villa further by guiding them into the upper reaches of the Premier League, but is still held in high regards with Hoops supporters and regards some of his best days in management being there.
"Celtic was an extraordinary experience and, had my wife not got ill, there would have been no reason to leave that football club," he said.
"I had some fantastic experiences and the 2003 UEFA Cup final against Porto in Seville was right up there with everything I have done in the game.
"I had the chance to work with some great players - Henrik Larsson, Chris Sutton, Neil Lennon and John Hartson to name just a few.
"Players who not only came to Celtic but attached themselves to the club.
"I listen to Sutton now who says his best days were at Celtic. Remember, this is a player who won the Premier League with Blackburn Rovers and cost Chelsea £10million.
"But I did have a great rapport with the crowds up there. However, it's like everything else - you only stay at a club if you're doing reasonably well."
O'Neill also reflected on his final days in charge of the club, speaking with regret after losing out on the title in the last game of the season to Rangers thanks to two late goals from then-Motherwell striker Scott McDonald.
"I did hope we could have kept it going. There was still plenty to achieve" he added.
"We lost the title on the last day when I was leaving the club. That was a real disappointment and beating Dundee United to win the Scottish Cup the following week didn't really make up for it."