Both the New Democrats and Progressive Conservatives knew about the changes at Ornge that shielded public money from government oversight, house leader John Milloy charged.
NDP Leader Andrea Horwath received a letter in December 2010 about the changes, which allowed Ornge to create a web of for-profit entities that were owned by its senior officers, he said.
It also contained details about their plans to borrow $275 million through bonds and buy more aircraft and a building to serve as its headquarters, Milloy said.
"It's very interesting that the honourable member stands here today and talks about ignoring documentation," he told the legislature.
Christine Elliott, deputy leader of the Opposition Tories, was also briefed by Ornge on all of their operations on May 19, Milloy added.
Last week, Ontario's auditor general released a scathing report criticizing the government for giving Ornge $730 million over five years with virtually no oversight of how the money was spent.
Despite numerous red flags, the government failed to monitor the agency as it used the money to make "questionable'' business deals, the auditor said.
But the opposition parties didn't sound the alarm either, despite their recent howls of outrage over Ornge, Milloy said.
"I ask ... when the NDP sat on that information for 15 months, why did they not raise questions in this house? Why did they not contact the minister of health?" he asked.
"It's passing strange the double standard that the member and her party are bringing to this debate in the legislature."
Horwath said the 2010 letter was actually Ornge's response to questions the NDP raised about how the agency was conducting its affairs. The party started filing freedom-of-information requests as soon as they received it, she added.
"The house leader and the government are simply trying to shift the blame from themselves onto opposition, which is just ridiculous really," she said.
"The reason the letter was issued was in response to the very questions that we were asking about Ornge ... I think that the government's playing some kind of game and that's not what people want."
Tory Leader Tim Hudak also dismissed Milloy's accusations, saying his party have been asking questions about Ornge for "some time" in the legislature.
"It's unfortunate to see that type of nonsense from the government when they should be tackling a very important issue," he said.
If the Liberals really want to clear the air about Ornge, they should abide by the will of the legislature, which passed a motion to strike a legislative committee on Ornge to restore confidence in the air ambulance service, Hudak added.
Milloy has shrugged off the motion, saying existing legislative committees will have ample opportunity to look at Ornge, including the public accounts committee which will have its first crack at Ornge on Wednesday.
The NDP and Tories fought back Monday by interrupting house business with motions for adjournment — one of the few procedural delaying tactics left to the opposition parties.
They also stepped up their calls for Health Minister Deb Matthews' resignation over the scandal.
Former Conservative health minister Elizabeth Witmer demanded Matthews' head Monday, saying some of her predecessors have stepped down for far less.
The Health Ministry received a draft copy of the auditor's report last September, when the provincial election was in full swing, but nothing was done, Witmer said.
"(Matthews) acknowledged last week that she didn't read the report in September because she was too busy campaigning; yet, she made several announcements as minister during the campaign," Witmer said.
"Apparently she could write cheques during the election but she couldn't oversee a corrupt agency that was putting lives at risk."
Premier Dalton McGuinty is standing by Matthews, saying he won't ask her to resign.
"I happen to believe that Minister Matthews has done a very good job in her capacity as the minister of health," he told the legislature. "As soon as she was briefed, she took concrete and substantive measures."
Ornge, which is currently under a police probe, has been mired in controversy for months over high executive salaries and allegations that public dollars may have been used for personal gain.
Following the auditor's report, the Liberals tabled new legislation that they say will bring more oversight to the agency.