Giving the ombudsman the authority to look into complaints against hospitals, ambulance services, nursing homes and retirement homes would rebuild the trust people have lost in the health-care system, she said.
"I don't blame people for not believing Liberal promises anymore," Horwath said.
"Their system of just throwing millions at a problem and saying, 'Just trust me' hasn't delivered results. It's given us eHealth, Ornge and other health-care scandals."
Ontario is the only province that hasn't introduced or passed legislation to give their ombudsman oversight of hospitals and nursing homes, Horwath said.
The Liberals' management of the massive health-care file has taken a drubbing over the past year. Ornge is under a criminal probe for questionable business deals and spending, while a lack of oversight over drug mixing has come to light after about 1,200 patients received diluted chemotherapy drugs.
An ombudsman would also help ensure that the Liberals meet her pre-budget demand for a five-day guarantee for home care, Horwath said. The Liberals have promised a five-day target, but agreed to spend substantially more than the NDP wanted to make it happen.
Ombudsman Andre Marin and his predecessors have long called for oversight of hospitals and nursing homes, said his spokeswoman Linda Williamson. He wrote to Health Minister Deb Matthews in March asking her to give him oversight of Ornge.
The minority government needs the NDP to help pass the budget and avoid an election. The Liberals addressed a long list of demands in the budget, but the New Democrats say they want more accountability.
Horwath has also asked for a financial accountability office, modelled on the parliamentary budget office in Ottawa. She's said there will be more demands next week.
But she won't "draw lines in the sand," she said Friday.
Premier Kathleen Wynne has called Horwath's proposal on the accountability office "an interesting idea," but said she had been unable to get a meeting with the NDP leader.
Wynne wouldn't say Friday whether she would acquiesce to the NDP's latest demand, but upped the pressure on the NDP to support the budget.
"One of the things that I'm hearing is people saying, 'What's going on here? Why can't we get a decision? You've got a budget. What's going to happen?'" Wynne said during a visit to Parry Sound, Ont.
"People are sort of, 'What are you guys doing?' So I want that certainty for the people of Ontario."
The Conservatives, who are pushing for an election, refused to say whether they'd give the ombudsman the same powers if they form the next government, even though it won't require more spending.
Tory Monte McNaughton said the province "has a dirty diaper" in the Liberal government that needs to be changed.
"I think there's never a problem with having more oversight across government... but we need to start at the top," he said.
"We need to change the team that's leading the province of Ontario."
Horwath said her consultations with the public over whether the NDP should support the budget will wrap up next week.
She couldn't provide a number of people who've weighed in, but said so far, it's in the tens or hundreds of thousands.