Kem Sokha said the party still wants meaningful election reforms, including within the National Election Committee, as well as a new election by 2016.
Legal NGOs have urged the National Assembly to allow input from civil society.
The National Assembly has received three draft laws on judicial reform that were approved by the Council of Ministers earlier this month.
President Barack Obama heads to Malaysia this weekend, where the host government's handling of the disappearance of the Malaysia Airways flight have drawn international attention - and criticism.
Prime Minister Hun Sen is currently on state visits to eastern Europe, and Sam Rainsy, president of the Cambodia National Rescue Party, is in western Europe.
Mu Sochua is leading a non-violent campaign by attempting to enter the park in protest of the government’s current ban on public gatherings.
Kem Sokha will end a two-week trip later this month, after meeting key US officials, including those in Congress, the State Department and the Defense Department.
Both sides had hoped to sign an agreement in front of the king on Friday, prior to the kick off of annual New Year’s celebrations.
Sam Rainsy told VOA Khmer Thursday he has not agreed to that date and has pushed for early elections in February 2016.
Cambodia’s electoral system may be in need of an overhaul, a leading rights advocate says.
The Rescue Party is in talks with the ruling Cambodian People’s Party to end a political deadlock in place since the 2013 elections.
Negotiations between both sides seeking an end to months of political impasse ended March 24 without full resolution of issues, though both sides say they are still open to talks.