The Republic of Ireland international, who previously linked up with Wayne Rooney during his time at Manchester United, is confident that Fletcher will get even better.
"I don't want to put any more pressure on him because he has enough on his plate, but he's definitely up there with some of the other strikers I have played with," the Shields Gazette quotes O'Shea as saying. "He's top class.
"I'd only played against Fletcher a couple of times before and he barely got a touch, but he scored two goals! When you see him in training you realise what a good player he is, not just as a finisher but his all-round game."
Fletcher moved to the Stadium of Light from Wolverhampton Wanderers in the summer.