The Scottish striker, 25, has already hit three goals in two Premier League games since his £14million arrival from Wolves.
O'Neill told talkSPORT: "I think he will score goals for us. I think he will be able to hold up play for us. I think that's really, really important for us, it's absolutely essential.
"He'll bring people into the game and I think during the next couple of months, when he does get properly fit, he's still struggling with that at the minute because of having no pre-season, he'll do really well.
"I mentioned people seem to have underrated him but not me. I watched Wolves play a lot last season and each time they played their games, despite the fact they might have lost some of those particular matches, I thought he was still excellent."