The swearing-in ceremony began at 10 a.m. ET with the 30 PQ members who won seats in Quebec’s general election on April 7.
CAQ's 22 MNAs and Québec Solidaire's three will be sworn in this afternoon.
PQ interim leader Stéphane Bédard was the first to take the two MNA oaths in the National Assembly’s Red Room, which had its Canadian flag removed for the occasion.
One oath pledges allegiance to the Queen and the other to the people of Quebec.
In a speech after his swearing-in, Bédard pledged that the PQ would be "respectful, vigilant and rigorous" in its duties as Official Opposition.
He promised the PQ's cooperation in working with all parties on initiatives to promote and protect the French language and Quebec's culture.
He also reaffirmed the PQ's commitment to sovereignty.
"This is the basis of our political commitment," he said.
The PQ led a minority government with 54 seats going into the election and had hoped to come out of it with a majority.
The majority went instead to the Liberals, who won 70 seats — a gain of 20 over their 2012 results.
The Liberal gains on April 7 came mostly at the expense of the PQ, including its leader Pauline Marois who lost her seat to Liberal rival Caroline Simard.
The Liberals were sworn in last Thursday, and the Liberal cabinet will be named and sworn in Wednesday.