Sunderland looked to be heading for their fourth defeat in a row on Saturday as they fell 2-0 down to the Cottagers, but Craig Gardner pulled one back from the penalty spot before Sessegnon fired home the equaliser 20 minutes from time.
The Benin international, who also found the target in the 2-1 defeat at West Brom the previous week, has demonstrated his versatility this term, playing as a support striker to Stephen Fletcher before shifting back to a wider role following the January arrival of Danny Graham from Swansea.
"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," O'Neill told reporters.
"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.
"When Danny came off, he went back into the position he's been playing most of the season and is equally adept in there as well."