Legislators from the opposition Cambodia National Rescue Party had refused to take their seats after alleging results of last year's general election were rigged.
The opposition reached a deal with Prime Minister Hun Sen and his ruling party two weeks ago for political reforms and the sharing of leadership positions in National Assembly committees.
On Friday, all 55 opposition lawmakers attended the session. Hun Sen's party holds 68 seats in the National Assembly. There are no other parties holding any seats.
Opposition leader Sam Rainsy reiterated his commitment to end the political crisis for the sake of the country.
"We will set up a new culture, a culture of dialogue ... to protect our country's integrity,'' he said. "We are going to end the culture of fighting with each other.
Hun Sen's authoritarian government has been in power for almost three decades and is known for intimidating its opponents. The opposition's strong showing in the July 2013 vote had one of the biggest challenges to Hun Sen's rule in years.