The 28-year-old Benin international on Thursday signed a new three-year deal which will keep him at the Stadium of Light until the summer of 2015 after weeks of speculation over his future.
O'Neill was delighted to have retained Sessegnon's services just days after staging a transfer coup to land Adam Johnson and Steven Fletcher. "I think he himself for the talent he possesses might think his goal ratio is not up there, but he is a quality player and he is hard to mark," O'Neill said.
"He had a great period last season - like the rest of us, he maybe tailed off a little bit towards the end when the Premier League status had been achieved and he got a little bit tired.
"But he's fresher now and I am delighted he is there and he is really necessary for us."
Sessegnon, who joined the Black Cats from Paris St Germain in January 2011, had been linked with a move back to his former club, while Arsenal were among those also to have been credited with an interest in him during the summer.
O'Neill admitted the uncertainty had been a worry, and was understandably delighted to have persuaded the player to commit his future to Sunderland. "I was naturally concerned, of course, but when he sees the likes of Fletcher and Johnson coming into the football club, I think that helps make your mind up a little bit," he said.
"It might not have had any bearing at all because Stephane very much keeps his thoughts to himself, but overall it's just very, very pleasing to have him, and when he returns to real proper sharpness, I am hoping he will be the force he was last season, certainly at least for three or four months."
While Sessegnon was ensuring himself on an extended stay on Wearside, one of his team-mates was edging closer to the exit. Kieran Richardson is in talks with Fulham over a move after the two clubs agreed a deal on Tuesday.
"You would be aware that at the moment, Kieran Richardson is speaking to Fulham and that may go through," O'Neill said. "If that goes through, then naturally we would be looking at someone in that position."
Source: PA
Source: PA