The 28-year-old had previously struggled to repeat his prolific form of last season, when he netted seven goals and assisted 12.
But Sessegnon has now scored in each of his last two matches for the Black Cats after moving out onto the flank following the January arrival of Danny Graham up front alongside Steven Fletcher.
"Stephane is right back to his best," O'Neill told reporters.
"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.
"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.
"He's great for us because he can play anywhere in midfield or down either side. At the minute, with Danny coming into the side, he's happy enough to go into a wide role."