Petrov has been a big hit at Villa Park so far this season, after joining the club for £8million in August 2006 from Celtic.
“Petrov was definitely inconsistent before. He had never really captured the sort of form that I had seen him display at Celtic – and I mean Celtic on European nights where he was fantastic,” O’Neill told Sky Sports.
“He has had to adapt. The role has changed for him here. At Celtic he was basically a midfield player getting beyond the centre-forwards – and I think I could score goals if I had Chris Sutton and Henrik Larsson playing up front when coming through from midfield!
“But here he is playing the holding role in the side. He is getting plenty of touches early on in games, his confidence is high, his strength is terrific.
“It is the sort of strength in holding people off that I’ve known Stiliyan have for five years at Celtic. This is what I’ve been pleased with and, when he gets up to head the ball, he is brave.
“Confidence is an amazing thing in football. His confidence was low last season and then it looked as if he was deliberating as to whether to go and challenge someone.
“Now confidence is so high, he is going in, winning the balls, he is setting things in motion and he can play himself. I really could not be more pleased with him. I thought he was outstanding against Sunderland.”