Jim Watson will be the next mayor of Ottawa with 10 new city councillors joining him around the council table.
Watson, who was considered the front-runner for the entire election campaign, secured the win with 49 per cent of the vote.
O'Brien comes in second with 24 per cent support; Clive Doucet is a distant third with 15 per cent.
"It's good to be back home," Watson said in a victory speech following his win.
"It appears the public voted for change and they voted for change in a very big way."
Watson told supporters he is committed to leading Ottawa by building consensus and seeking common ground around the council table. He adds that hard work starts the day after the election.
O'Brien congratulates his rival; Doucet moves on
O'Brien picked up the phone to congratulate Watson on his win Monday night, saying he plans to give Watson his full support moving forward.
"I told Jim tonight, it's been the most exciting experience of my life to be mayor of a city this big, this diverse, and quite frankly this wonderful," O'Brien said.
Earlier, he told supporters that election campaigns are like wars – they're a difficult battle to be won.
Meanwhile, Doucet failed to capitalize on the momentum his team felt going into Monday's vote. He said he's proud of his work at the council table and it's time for him to move on.
Ottawa elects 10 new councillors
Along with the mayor, the face of council is also changing. Four incumbents didn't run for another term, while six others lost their seats on council.
In Bay Ward, Coun. Alex Cullen was defeated by Mark Taylor who led by more than 1,000 votes. Cullen was an early entry into the mayor's race, and then dropped out to try to reclaim his seat on council.
In Kitchissippi, Katherine Hobbs defeated Coun. Christine Leadman by about 500 votes. In Cumberland, Stephen Blais took Coun. Robert Jellett out with more than 1,000 votes.
In Rideau-Goulbourn, Scott Moffatt won the seat over Coun. Glenn Brooks by 2,500 votes. In Rideau-Vanier, rookie Mathieu Fleury narrowly defeated Coun. Georges Bédard. In Beacon Hill-Cyrville, Coun. Michel Bellemare was also narrowly defeated by Tim Tierney.
Meanwhile, David Chernushenko won the seat in Capital Ward that was left empty when Doucet entered the race for mayor.
Chernushenko, who is familiar to losing federal battles with the Green Party, started his victory speech with a joke: "Does anyone know how to make a victory speech?"
He vows to bring a new vision to Ottawa, work on sustainability issues, and re-work the deal to redevelop Lansdowne Park.
In Rideau-Rockcliffe, former regional chair Peter Clark took away the win after the seat was left empty by Coun. Jacques Legendre.
Keith Egli will be the new councillor for Knoxdale-Merivale, which was vacated by Coun. Gord Hunter who chose not to run again. Coun. Peggy Feltmate's old ward of Kanata South, now belongs to Allan Huble.
City officials say unofficial results show 45 per cent of eligible voters cast a ballot in this year's election. That number is down from a 54 per cent voter turnout in 2006. The official numbers are expected to be released on Thursday.
Ottawa's new city council will be sworn in on Dec. 1.
