The budget addressed many NDP concerns, admitted party Leader Andrea Horwath, but did not include "a key ask" to make government more transparent by creating a financial accountability office, modelled on the parliamentary budget office in Ottawa.
"This independent commissioner would be a spending and revenue watchdog of the government and its agencies," said Horwath.
"We need to ensure that there are no more Ornge, eHealth or gas plant scandals in this province. Something needs to bring an end to the Liberals' free ride."
The Progressive Conservatives rejected the call for an accountability officer, saying the province does not need more bureaucrats.
"If you think a government is corrupt and can't be trusted with taxpayer dollars, the answer is not a new bureaucracy, it's a new government," said PC Leader Tim Hudak.
Horwath said she wouldn't make the accountability officer a deal breaker for supporting the minority government's budget because she's still consulting the public and expects to make more demands.
"Am I going to draw a line in the sand at this point? No," she said.
"I think there are other things that are coming forward that people are talking to us about that are ideas we're going to be sharing as well. It's all about making sure that this budget is accountable and balanced."
The Liberals addressed a laundry list of NDP concerns in the budget, with promised cuts to auto insurance premiums, increased welfare rates and more money for home care services.
Premier Kathleen Wynne called Horwath's proposal "an interesting idea," but said she had been unable to get a meeting with the NDP leader.
"I need to have a meeting with her," Wynne said in Waterloo, Ont.
"The budget is not a starting point for negotiations. I think it should go forward the way it stands. We're not starting from scratch."
Wynne repeated that she's not afraid of what she calls "an unnecessary election" if the opposition parties make the choice to defeat the budget.
Horwath isn't saying how many members of the NDP caucus are pushing her to forget about the budget and force an election, and said the public feedback has been split on the idea of defeating the Liberal government.
"They elected a minority government on purpose. They want us to try to achieve things for them," she said.
"We're happy to see many of our ideas reflected in the budget, but we want to actually make sure those things get achieved."
The Conservatives, meanwhile, moved a notice of motion Wednesday to try and force the legislature to debate its non-confidence motion filed over the $585 million spent to cancel gas plants in Oakville and Mississauga.
The Tories call it an "affront to democracy" for the Liberals to refuse to call the "want of confidence" motion for a vote, so they're using an opposition-day motion to force the NDP to take a stand on the gas plants or to keep propping up the government.
Wynne said the budget is the best time for the opposition parties to express confidence, or a lack of it, in her government.
PC house leader Jim Wilson said the Liberals "already bought that vote" by caving to the NDP's budget demands.
Horwath refused to say if she would support the PC opposition-day motion.
"We'll debate the motion when it gets on the floor," she said.