Attacker Sessegnon has scored a goal in each of the club's last two encounters, which in turn has helped them begin to move away from relegation trouble under the guidance of new manager Paolo Di Canio.
"Ever since I came to the club I've known how important Sessy is to the team," O'Shea told the club's official website. "Once we get him the ball in the right places he's going to hurt teams.
"To be fair to Sessy, the work he has been putting in in training in the last two weeks has been fantastic.
"I've seen him do it in training and his goal earlier in the season against Fulham was like that, he's more than capable of scoring from outside the box and hopefully there is a few more to come from him this season."
Sessegnon joined the Black Cats from Paris Saint-Germain in 2011.