The Black Cats have struggled this season and have relied heavily on the Scotland international's goals to steer them away from the bottom three.
Fletcher has been struggling with an ankle injury which saw him miss the recent clash with Chelsea, and limited his performance at Manchester United, but is now raring to go.
O'Shea told reporters: "Thankfully we have got Steven Fletcher. I don’t know where we would be without his goals.
"He came here with a bit of a price-tag on his head, he settled in straight away, he’s a great lad who works hard. The manager has said he is a quiet lad who doesn’t say much but as long as he is scoring goals he can stay as quiet as he likes.
"His finishing is top drawer as he has shown with his goals. They have been really top class finishes. He has had to deal with having a big price-tag in his next few moves but that has not affected him one bit.
"Fingers crossed we keep him fit as much as we can. He has had a few little niggles but nothing major, thankfully. It’s up to the rest of the lads now.
"I’m sure Steven himself will want to improve and he can do better. It’s about the whole team and squad – especially when the manager makes or two changes, which he will during this period."
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O'Shea has lamented a tendency to concede early goals this season and hopes that, with improved performances, the Black Cats can emulate Norwich's recent success and shoot up the Premier League table.
He added: "We keep talking after games about finishing well, but we have zero points from them. We can’t keep saying that and we have to make sure we do our jobs for the 90 minutes. We can’t keep doing what we doing or the end of the season will come round very quickly.
"The season has been stop-start. You think you are getting yourself going, then you suffer a defeat you’re not expecting and there’s a lot of room for improvement. There definitely is. We are not incapable of doing it.
"The top ten is not far away. We have seen where a team like Norwich have shot up the table with a good run and we are more than capable of that."