In the 1996 provincial election Ottenheimer defeated Liberal incumbent Hubert Kitchen in the district of St. John's East, winning the seat for the Progressive Conservatives. His win came despite the Liberals winning an overwhelming majority government and the PC Party losing seven seats from the previous election. Ottenheimer was easily re-elected in the district in the 1999 general election. From 1996 to 2003 Ottenheimer served in opposition and was a critic for a number of ministries. Ottenheimer had considered running for the party's leadership following the 1996 and 1999 general elections, but did not enter either race.
Minister of Education
Ottenheimer was re-elected in the 2003 provincial election and the Progressive Conservatives won a majority government under Danny Williams. Weeks after the election Williams appointed Ottenheimer as the province's Minister of Education. In February 2004, Ottenheimer confirmed that the provincial government would maintain the post-secondary education tuition freeze. The PC Party had committed to maintaining the freeze in their 2003 election platform but following a review of the province's finances the new government began to make sweeping budgetary cuts.
Minister of Health and Community Services
On October 1, 2004, Ottenheimer was named minister of Health and Community Services, succeeding Elizabeth Marshall who quit the post after a public dispute with Williams over his management style. In October 2005, it was reported that Eastern Health, the province's largest health authority, was reviewing breast cancer tests dating back to 1997. The health authority later announced that between 1997 and 2005 over 400 women received inaccurate test results. The incident led the provincial government to call a judicial commission of inquiry into the conduct of Eastern Health. The Cameron Inquiry began in May 2007 and finished hearing testimony in October 2008. Ottenheimer was questioned at the inquiry in March 2008, about his role in the matter. Prior to Ottenheimer's testimony, it was reported that he had found out about the error in the testings on July 19, 2005, several months before it was reported in the media. Ottenheimer stated that he had wanted to go public with the information when he found out but was advised by officials at Eastern Health not to. In March 2005, Ottenheimer blacked out on an airplane en route to Gander and had a pacemaker installed later in the day. The incident led to Ottenheimer taking a two-month leave of absence from cabinet.
Minister of Intergovernmental Affairs
On March 16, 2006, Ottenheimer was appointed Minister of Intergovernmental Affairs. The move to the less demanding portfolio was a result of the health issues he faced the previous year. Ottenheimer said he needed a lifestyle change and had spoken to Williams the week prior to the shuffle about his health. On June 15, 2007, Ottenheimer announced he would not be seeking re-election in that year's provincial election.
Federal politics
On March 30, 2011, Ottenheimer announced that he would be the Conservative Party of Canada candidate in the district of Random—Burin—St. George's in the 2011 federal election. He was defeated by Liberal incumbent Judy Foote, receiving 8,322 votes.
Leadership bid
Ottenheimer unsuccessfully ran for the leadership of the provincial Progressive Conservatives in 2014 losing to Paul Davis. He subsequently decided against running for the federal Conservative nomination in Avalon for the 2015 election. Davis subsequently appointed Ottenheimer to head the Newfoundland and Labrador Housing Corporation. He was termination without cause from the NL Housing Corp. in 2016 following the Liberals forming government. In 2018, he returned to law joining the Morrow, Morrow & Crosbie firm in Bay Roberts.
