After the 1970 leadership convention, Stephen Lewis became leader, and guided the party to Official Opposition status in 1975, the first time since the Ontario CCF did it twice in the 1940s.

The NDP's predecessor, the Co-operative Commonwealth Federation (CCF), was a democratic socialist political party, founded in 1932.

[6][7] The Ontario CCF Council ceased to exist formally on Sunday, 8 October 1961, when the newly elected NDP executive officially took over.

The charismatic and dynamic Lewis ran a strong election campaign that forced the Tories to promise to implement the NDP's rent control policies.

[9] Hopes were high that the NDP was on the verge of taking power, but in the 1977 provincial election, the Tories under Bill Davis again won a minority government.

Cassidy's policy advisor in the leadership campaign was James Laxer, a former leader of The Waffle NDP faction which Lewis had expelled from the party in 1972.

When the accord expired in 1987, Premier David Peterson called an early provincial election and the Liberals were re-elected with a large majority.

Under Rae, the NDP ran a strong campaign, which was also aided by a successful showing for federal New Democratic Party a couple years earlier.

As a result, the NDP won a large majority government of 74 seats while the Liberals suffered the worst defeat in their history.

In government, the NDP disappointed supporters by abandoning much of its ambitious program, including the promise to institute a public auto insurance system.

Rae's government passed employment equity legislation and amended the province's labour law to ban the use of replacement workers during strikes, but this did not win back union support.

All 10 of the federal NDP's Ontario MPs lost their seats to Liberal Party of Canada challengers by large margins.

The official opposition Ontario Liberals under Lyn McLeod were initially the beneficiaries of the NDP's unpopularity, but their poor campaign saw the momentum swing to the resurgent Tories under Mike Harris, who vaulted from third in the legislature to win a large majority.

Shortly after the 1999 provincial election, Hampton cited the Swedish model of social democracy as closely reflecting his own beliefs.

The Progressive Conservatives had reduced the size of the legislature, so provincial ridings now had the same boundaries as the federal ones, and so the official party status threshold was lowered.

[16] As well, the Public Power Campaign also dealt with rolling-back the social program cuts from the Harris government's Common Sense Revolution.

[19] Several unions, such as the Canadian Auto Workers (CAW), promoted strategic voting to their membership and the public, which further added to the party's woes.

[20] The newly elected Liberal government offered to give the NDP caucus research funding if their members agreed to sit as independents.

This tragic event, in conjunction with a recent and unpopular tax increase by the Liberals, provided the NDP with an opportunity to regain party status.

Some opposition sources believed the Liberals, mindful of their humiliating defeat to Horwath, had loosened their interpretation of the rules so that whoever ran for the NDP in Toronto—Danforth couldn't use the threat of lost status in a campaign.

This issue became moot when, on 30 March 2006, NDP candidate Peter Tabuns won the by-election in the Toronto—Danforth riding by a 9% margin over the Liberals' Ben Chin, alleviating another party status crisis.

[24] In the riding of York South—Weston, adjacent to Parkdale—High Park and once the seat of former leaders Bob Rae, Donald C. MacDonald and Ted Jolliffe, the NDP continued its string of recent by-election successes by taking away another Liberal stronghold.

[citation needed] France Gélinas also successfully retained the riding of Nickel Belt, following the retirement of Shelley Martel.

[38] At the official televised leaders' debate, her political rivals criticized the Ontario NDP's handling of the economy in the early 1990s, but Horwath further distanced the party from Rae by pointing out his current allegiance to the federal Liberals as interim leader of the (federal) Liberal Party.

[41] In September 2012, NDP candidate Catherine Fife won a by-election in the riding of Kitchener—Waterloo after the resignation of former Progressive Conservative MPP Elizabeth Witmer.

Further by-election victories in ridings formerly held by the Liberals included Peggy Sattler in London West and Percy Hatfield in Windsor—Tecumseh in August 2013, and Wayne Gates in Niagara Falls.

[44] After the interim leadership of Peter Tabuns, Marit Stiles was declared Ontario NDP leader by a majority vote at an event in Downtown Toronto on February 4, 2023.

[45] In October 2023, Hamilton Centre MPP Sarah Jama was removed from the NDP caucus for allegedly failing to abide by the terms of an agreement between herself and Stiles.

Jama subsequently apologized for her posts but, in defiance of the party's directive, refused to remove the statement, instead pinning it to the top of her feed on X.

The convention is made up of delegates elected by riding associations, sections of the party (ONDY, Women's, LGBT, Ethnic, Aboriginal, Disability), affiliates such as labour unions and other bodies.