From then to 1982, he was the general manager of reliability-testing firm Reltek Inc. in Kanata, subsequently leaving to open Calian Technologies Ltd., a staffing (outsourcing) and engineering service provider.

[2] In July 2006, O'Brien announced he would be running for the office of mayor of Ottawa in the 2006 municipal election, calling himself a centrist candidate.

[7] He was critical of what he saw as secrecy of some of the elements of the project, as well as the effects the construction would have in areas such as the Albert and Slater street corridors, and the removal of some of the existing express bus services.

In September 2006, O'Brien wrote a letter to the federal President of the Treasury Board, John Baird, requesting a formal review of the project.

[8] During a candidates' debate on September 13, O'Brien stated that one solution to the problem of homeless youth sleeping under a bridge was to replace them with a tourist kiosk.

O'Brien's other campaign commitments on law and order included his promise to eliminate the city's free crack pipe program.

During the Ottawa mayoral campaign, O'Brien had promised a no-tax hike policy; however, due to a larger shortfall than anticipated, he said that he could not guarantee no tax increases.

After several budget meetings, O'Brien criticized several councillors for supporting a tax increase within the rate of inflation and for not taking the procedures seriously.

[14][15] On February 26, 2007, news sources reported that the City, while recuperating surplus and reserve funds and raising various user fees, would not increase property taxes that year.

Despite his previous support for a tax freeze, O'Brien proposed a 2% tax-levy until 2010 for infrastructure projects, which would give an additional $180 million in revenues to the City.

[26] O'Brien and much of the Council had stated prior to the budget talks that cuts to essential services such as community centers, libraries and transit were unacceptable.

Proposals to combat the shortfall included a 4.9% tax increase and cuts to as many as 230 jobs, 700 daycare spaces, $4 million in arts program, and transit service.

His strategy was met with concern by several councilors, organizations, and other prominent political figures, with former mayor Marion Dewar insisting on a degree of public consultation.

The heavy snowfall caused a major budget shortfall for snow removal operations, with some cost estimates as high as $23 million.

[33] As it opted for eliminating the deficit by using provincial infrastructure funding announced in the 2008 budget and money from the sale of Telecom Ottawa, the proposal was met with heavy opposition from many citizens and councillors, and it was defeated during a City Council meeting on March 26, 2008 .

On December 6, Ottawa Council voted 12–11 in favour of expanding the O-Train, except the section that would travel through downtown, while adding an environmental assessment that would study the possibility of building tunnels beneath Albert and Slater streets.

[38] On November 28, 2007, while facing a $280 million lawsuit from Siemens, the contractor for the original north-south project, the City Council approved a new transit plan worth just under $2 billion.

[40] A future section near Westboro requires more studies due to concerns by the National Capital Commission and area residents on using light-rail on the Kichi Zibi Mikan.

The parties agreed to send the dispute issues, such as sick days, work-rest rules and other benefits, to binding arbitration.

O'Brien's platform stated that he wanted to "give the Mayor, rather than staff, the authority to bring forward the city’s budget plan for debate.