The Black Cats boss admits Gardner's strengths suit a more advanced role, but his is happy that his versatility has allowed him to plug a gap which otherwise might have become a problem.
O'Neill said: "He is a midfield player and he is capable of scoring goals from midfield, that's one of his big strengths.
"He is one of the players capable of scoring double figures for us in the Premier League.
"He has got a fantastic shot with no back-lift. He hits the ball as hard as anybody in the Premier League and obviously, the closer he is to goal, the better chance he has of doing that, outside free-kicks and things like that.
"But the very fact that he can play in a number of positions is good news for us."