The 26-year-old playmaker joined the Black Cats in January 2011 and was particularly impressive during his first full Premier League season last term, scoring six goals in 24 appearances.
That form has led to speculation linking Sessegnon with a move away from the club but O'Neill has highlighted his importance to the Mackems' future.
"There's no doubt Stephane Sessegnon is a player that we obviously want to keep and we'll be doing everything we can to do so," he told the Sunderland Echo.
"He adds something to our attack that no other player does and our priority is to improve ourselves as an attacking force next season.
"If you want to do that, you have to look at keeping your best attacking players and attracting others.
"Stephane has been making the right noises about being happy to stay at Sunderland and hopefully that will be the case."
However, the Northern Irish coach stopped short of commenting on whether the Benin international would stay at the club, citing the unpredictable nature of the summer transfer window.
"I don't want to say something now which will be thrown back at me next season with people saying: 'Well you said this, or you said that'," he added.
"You can never tell what's going to happen over the summer and I don't want to be saying anything categorically at the start of it which will make us look foolish at the end of it."