The 28-year-old scored seven goals for the club last season, but has largely struggled to produce on a regular basis in this campaign.
However, the playmaker has now scored in his last two matches, with O'Neill revealing his delight at Sessegnon's recent performances.
"Stephane is right back to his best. I said a few weeks ago if he could add some goals to his game he would be priceless and he's doing precisely that. He is playing brilliantly at this minute and was very dangerous throughout the game," O'Neill told reporters at a press conference.
"He is going past players and has regained his confidence, which is a big thing for him. I think the couple of goals he's scored have been a big help in that. Although, in fairness, he was flying last week before he actually got his goal.
Next up for Sunderland is an away match with Queens Park Rangers on Saturday.