After a slow start to his Villa Park career, club captain Petrov is now proving his value.
O'Neill told Villa's website: "It's taken some time, I accept that.
"But interestingly when he was at Celtic we had a holding midfield player in Neil Lennon there, who did all the work that Stiliyan now does for us.
"It allowed Stiliyan to get forward and score goals. When you've got centre forwards like Henrik Larsson, Chris Sutton and John Hartson up there, it was a midfield player's dream to get forward.
"But now he has come here, it took him some time to adapt, which was a real shame. But I am absolutely delighted that Villa fans are now seeing the very best of him in a somewhat different role.
"I think that he feels, other than scoring the occasional rocket against Derby County and one against Stoke, that perhaps maybe he might be able to get a few more goals.
"But I couldn't be more pleased with him. He's been very strong, very assertive and an excellent player with the ball."