Oswald filed her nomination papers at party headquarters, two months after she and four other senior ministers suggested Premier Greg Selinger consider resigning amid sagging opinion polls. The move prompted Selinger to call a leadership contest for March 8 at the party's annual convention.
Oswald said going up against Selinger is nothing personal. She said she offers the best chance for the NDP to revive its popularity in time for the next election, slated for April 2016.
"Every political party in Canada, of every political stripe, needs to go through a process of renewal, and we are no different," she said.
"I'm entering this race to offer an alternative vision and perhaps a renewal for what our party can mean going forward."
Oswald also said she is open to reversing a provincial sales tax increase that angered voters. The government raised the tax to eight per cent from seven last year, prompting an immediate downturn in opinion polls. The government said at the time the tax increase would only be for a 10-year period to fund infrastructure programs.
Brian Pallister, leader of the Opposition Progressive Conservatives, has promised to reverse the tax increase, if he is elected, sometime within his first mandate.
Oswald said Friday the tax timelines proposed by the NDP and Tories show the parties are not that far apart.
"With only a two-year gap between us, I think that there's a lot of room to move and I would be very open to looking at that."
Selinger has said he plans to stay on and fight to keep his job. He said earlier in the week he planned to file his nomination papers sometime before the party's Jan. 6 deadline.
Oswald, a popular New Democrat who took the suburban Seine River seat from the Tories in 2003, has worked as minister of health and, more recently, as minister of jobs and the economy.
She voted in favour of the sales tax hike, along with all other New Democrats, and helped promote it as a way to fund roadwork and other repairs. But she has since tried to distance herself somewhat from the decision by saying a vote against a budget bill in the legislature can cause a government to fall.
"I was not going to vote against my government and bring down my government at a time when the Conservatives would have a (chance) to come in."
Oswald has attracted the support of some top-level New Democrats. Anna Rothney, a top adviser to Selinger, has taken a leave of absence from that post to play a "leadership role" in Oswald's campaign, Oswald said. Eugene Kostyra - a former cabinet minister, adviser to former premier Gary Doer and campaign adviser to Selinger in 2011 - is also backing Oswald.
Also backing Oswald is Nancy Allan, a former education minister who Selinger demoted from cabinet in 2013. Allan sent out emails inviting people to Oswald's public campaign launch at the a community centre Sunday afternoon.
"I think Theresa is, without question, our best hope for leading us into the next election," Allan said.
"I don't want to see everything that we have worked for be taken away."
As education minister, Allan passed a law requiring schools to accommodate students who want to establish gay and lesbian support groups, over the objections of the Tories. Allan pointed out that a recently nominated Tory candidate, James Teitsma, was a director with a religious lobby group that tried to stop the legislation.
No other candidates have entered the NDP leadership race so far. Infrastructure Minister Steve Ashton, who ran against Selinger in the 2009 leadership contest, has not ruled out another try.