In October 2016, Oosterhoff was nominated as the Progressive Conservative candidate in Niagara West—Glanbrook to replace Tim Hudak, who resigned his seat that September. Former MP and current president of the Progressive Conservative Party of OntarioRick Dykstra was the favoured candidate by both the party establishment and PC leader Patrick Brown, but Oosterhoff had more local support, including that of his church. Social conservatives such as Charles McVety accused Brown of muzzling Oosterhoff during the campaign because of the candidate's opposition to the new sexual education curriculum. On November 17, 2016, Oosterhoff defeated New Democratic challenger, Mike Thomas, by 9,528 votes. He was sworn in as an MPP on November 30, one day afterNathalie Des Rosiers, who also won a byelection on November 17, was sworn in. Critics said the delay was to keep Oosterhoff out of the legislature for a vote on Bill 28 which was unanimously passed on November 29. Bill 28 made it easier for same-sex couples to become legal parents, and Oosterhoff had previously voiced objections to the bill saying he "definitely would not have supported it" and that it was "disrespectful to mothers and fathers". Brown said the delay was so that Oosterhoff could organize a celebration party for his family and supporters. Oosterhoff is a social conservative, and believes that abortion and same-sex marriage should be illegal. However, in 2016 he stated that he is “absolutely not” a homophobe. On March 7, 2017, Oosterhoff defeated PC vice-president and Niagara Regional Councillor Tony Quirk for the PC nomination for the next provincial election in the new constituency of Niagara West, which replaced Niagara West—Glanbrook as a result of federal redistribution. Oosterhoff picked up 24,361 of the vote in the 2018 Ontario general election, defeating NDP candidate Curtis Fric. On June 29, 2018, Oosterhoff was appointed to the role of Parliamentary Assistant to Lisa Thompson, the Minister of Education. Oosterhoff was widely criticized on April 20, 2018 for his televised rendition of African-American spiritual 'Nobody Knows the Trouble I've Seen' given his socially conservative views and trouble-free middle class upbringing. On October 31, 2018, Halloween, Oosterhoff celebrated the passing of Bill 4 Cap and Trade Cancellation Act with a rendition of "Monster Mash" at Queen's Park.