The Cambodia National Rescue Party or CNRP says it has struck a deal with the Prime Minister Hun Sen to end the political crisis and to begin reform.
Cambodia has seen a crackdown on public meetings and dissent, with a rally by striking workers in January leaving four dead.
And eight CNRP politicians were arrested after a demonstration, last week, to face insurrection charges - punishable by 30 years in prison.
Opposition leader and CNRP President Sam Rainsy explained to what he has agreed to.
Correspondent: Karon Snowdon
Speakers: Sam Rainsy, Cambodian Opposition leader
RAINSY: We wanted the democratisation process to be put back on track, because it had derailed. And with the concessions that we have obtained from the ruling party, we have hope now that things would get better, therefore we accept to play the game and to take our seats at the National Assembly.
SNOWDON: The phone is difficult. Can you very briefly outline what concessions have been agreed to?
RAINSY: It's the reforms of the Electoral Commission, which has been so far under the control of the ruling party. This is why every election in Cambodia has been rigged and now with the reform, especially the composition of this very Electoral Commission, we have assurances that the next election, any future election, would be held in a better way.
SNOWDON: So you've agreed to basically accept the outcome of the last election, which you quite strongly disputed and has kept you out of the Parliament for a whole year now?
RAINSY: Yes, but in exchange for significant and promising concessions of guarantees, of reform, we are pretty sure that the next elections would reflect the winner of the people and then we definitely will have the upper hand.
SNOWDON: And what of the jailed members of your party - has any agreement been reached that will change their circumstances?
RAINSY: Yes, they are released today. This is a part of a comprehensive political settlement, because the very fact that we joined the National Assembly means that all the National Assembly members of parliamentarians will get parliamentary immunity, therefore they will be immuned from any prosecution, so we don't have to worry about that now.
SNOWDON: So will they be immediately free? They were facing 30-years in jail if they were convicted, so they will have no charges and they will not face jail?
RAINSY: No, no. They will be freed today.
SNOWDON: You are hopeful of change, but I would suggest that there is a large compromise on your part, by now accepting your seat in parliament. Could it be said that the government has, in a sense, blackmailed you by holding the party members, the members of your party, harassing them and threatening them with 30 years in prison. Do you see that as a form of blackmail, if I can put it as bluntly, as that?
RAINSY: Yes they have tried, they have tried to intimidate us, but it is not for this reason. We have not given in anything. We have obtained what we have asked for, even before the arrest of our parliamentarians.
SNOWDON: And what do you now hope you will be able to achieve or how effective do you think you will be, as a working Opposition, taking your place within Cambodia's Parliament now? What will you be working for, besides that electoral reform, of the Electoral Commission?
RAINSY: We definitely will have a say in parliament, because this is the first time that there is a united democratic Opposition in Cambodia.
And actually, the two parties, the ruling party with the United Democratic Opposition. We represent about the same strength, with the same popular support. This is inspite of election rigging. Had the election been free and fair, we would have won an outright victory.
But we are patient. We cannot obtain everything at the same time. We have left the ground now for a democratic change in the near future.
SNOWDON: When does Parliament resume?
RAINSY: In the next few days, after we swear in and then we will take up our seats next week or in a couple of days.