Stephane Sessegnon on Thursday signed a new three-year deal with Sunderland which will keep him at the Stadium of Light until the summer of 2015 after weeks of speculation over his future.
The three men will join forces in what the Ulsterman hopes will be a much more attack-minded side, and one in which he can build upon the talent he has already demonstrated at the Stadium of Light, where he was leading scorer with eight goals during the last campaign.
"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 toward 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 Saint-Germain in January 2011, had been linked with a move back to his former club, while Arsenal was among teams with an interest in him during the summer.
O'Neill admitted the uncertainty had been a worry, and was understandably happy to have persuaded Sessegnon 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 an extended stay at Wearside, one of his teammates was edging closer to the exit.
Kieran Richardson is in talks with Fulham over a move after the two clubs agreed to a deal on Tuesday.
The Black Cats are already engaged in the hunt for a replacement ahead of Friday night's transfer deadline with midfielder Jack Colback having lined up at left back in Tuesday night's 2-0 Capital One Cup victory over Morecambe in the 27-year-old Richardson's absence.
"I think Kieran might have the option of playing further forward for Fulham, which might be his preferred position, but if he did go, then we would be looking at something and I am pretty confident we would be able to resolved that," O'Neill said.
"Jack played exceptionally well there the other evening - he can play in a couple of positions for us - but I still think we would need somebody to fill that."