The former Everton forward, who spent a loan stint at Tottenham Hotspur last season, finalised a switch to the Stadium of Light yesterday on a one-year deal.
"I'm not expecting him to play every single game," said O'Neill. "That would be an enormous ask for him, but in the course of time I think he can help us.
"Despite Saha coming in we're hoping to still strengthen and we want to do because it's a long season."
The Black Cats go up against Arsenal in the opening day of the new Premier League season tomorrow.