Ceci recovered a clearing attempt by the Blues and sent a wrist shot towards the St. Louis goal that beat Brian Elliot at 3:59 of the extra period.
Jean Gabriel Pageau and Bobby Ryan scored in regulation for the Senators (14-15-6), who picked up points for the fifth time in their past six games.
Roy also took a hooking penalty in overtime but the senators were unable to take advantage of the four-on-three.
Ryan scored a highlight-reel goal on a pass from Kyle Turris, who put the puck on Ryan's tape from 40-feet away to right in front of the St. Louis goal.
Ryan cut in front, went to his backhand and beat Elliott to tie the game 2-2. It was also the only goal of the third period and sent the game to overtime.
Goaltending played a key role throughout the game as former Senator Elliott duelled with Ottawa's Lehner. Each made some big saves, especially through the first two periods.
Pageau beat Elliott early in the game as he found the back of the net over the Blues goalie's glove. Ryan set up the play from behind the net, finding Pageau in the slot.
That goal at 3:37 of the first stood up as the only goal until just 3:10 remained in the second when Stewart scored his first of two goals.
Roy made a long breakout pass to Stewart, who slid a backhand along the ice and through the legs of Lehner at the end of a breakaway.
With just 12 seconds remaining in the second, Brendan Morrow's miscue led to Stewart's go-ahead goal.
While behind the net, Morrow fanned on a pass attempt out front but the puck went directly to Roy at the side of the goal. Roy's no-look backhand pass was met with no hesitation as Stewart delivered a one-timer to the far side past Lehner for a 2-1 St. Louis lead.
Notes: Coming into Monday's game the Senators were 1-9-2 against the Western Conference while the Blues were 11-2-0 against the Eastern Conference…Scratches for the Senators Monday were defencemen Marc Methot and Eric Gryba along with forward Matt Kassian. The Blues sat forwards Jaden Schwartz and Vladimir Sobotka plus defenceman Ian Cole…Senators coach Paul MacLean was drafted by the Blues in 1978. He was traded to Winnipeg before returning to St. Louis to play his final two seasons. MacLean played 115 games for the Blues and had 40 goals and 44 assists…Ottawa is the only team the Blues have not shutout in club history.