The 60-year-old welcomed the Benin international's recent insistence that he will stay on Wearside, despite rumours of interest from Arsenal and former club Paris Saint-Germain
The 27-year-old has enjoyed a productive first full season with the club, notching seven goals and providing nine assists in 27 league appearances.
The Benin international this week moved to quell suggestions he is set to depart the Wearside club amidst rumoured interest from Arsenal and his boss has welcomed the news."I didn't know he had said anything publicly, I must admit. I didn't know he had spoken, the oracle, and I am delighted he has done," O'Neill told reporters.
"He won the player of the year at the meeting of the supporters' club, and I think he was genuinely pleased with that.
"The trophy that he picked up is about four inches bigger than he is. He seemed in decent form."
Despite the recent fan recognition received by the forward, O'Neill was keen to keep his player's feet on the ground.
"If he puts in performances like he put in last week [against Bolton], then we might have some other thoughts on [him staying] - he was rubbish," he continued.
"But he's been a terrific player for us, really terrific, and I am delighted with him.
"Obviously, we have to try to keep the best players. The only way for us to progress is to keep our best players, and Sessegnon definitely comes into that category."The Northern Irishman also went on to reveal that he is unsure over the fitness of defender Titus Bramble ahead of Sunday's clash with Fulham at Craven Cottage.
"Titus has trained for about the last 10 days with us and he has had no ill effects, which has been good," he added.
"Last week he had just trained a few days so I said would give him an extra week, but each day he has trained there has been no reaction, so that's progression for us."
"We have got two games to go. He is doing very well. Let's say if there was a month left in the season, I would have genuinely had no doubt about it, if he continues the sort of progress he is making.
"The two games might be a bit tight, but I am not sure. He is feeling much, much better each day."