Young hobbled off the pitch at the end of Villa's 4-1 demolition of Newcastle after being on the receiving end of a series of committed and fierce challenges from the Magpies defenders.
O'Neill said: "Ashley came limping off against Newcastle, with a bad back, sore knee, sore everything, but he had done his work by then.
"But what a player he is. He is coming off the pitch in bits but what a player. He has taken the Premier League by storm and has now got into the England squad on the back of that.
"I actually think Ashley enjoys being a marked man.
"It means he is making an impact on the game. His confidence is high and the crowd love him. He loves taking people on and there is no doubt we are a lesser team without him.
"He's a class player. The great thing about his own self-belief and self-confidence is that teams are doubling up on him and ignoring other players.
"It is not just the ability to pick the ball up and go 60 yards quickly with it. It is the ability to beat players on the edge of the box and the ability to deliver from open play or set-pieces.
"But also to work back in the manner he does is fantastic."
Striker John Carew said: "As a forward, you can only reap the benefits when you have someone like Ashley putting such great crosses into the box from corners or free-kicks.
"It invites you to go in there and attack the ball and we have scored so many goals from set plays this season.
"Ashley is getting a lot of attention and teams know about his skills now but he is still doing great things for us, great performances for us, and he is still important for our game."