The High Negotiations Committee (HNC) proposed holding six months of negotiations with President Bashar al-Assad, accompanied by a full ceasefire.
Mr Assad would then hand over power to a unity government that would run Syria for 18 months and organise elections.
Syria's deputy foreign minister later insisted Mr Assad would not step aside.
Faisal Mekdad told the BBC that demands that the country's elected leader be removed were "crazy" and "unbelievable".
"We are saying, let the Syrian people decide their own fate. Their own future without intervention. Don't make preconditions who will rule Syria," he said.
Mr Assad has consistently refused to step down since the beginning of the war, which has left more than 250,000 people dead, and correspondents say the recent battlefield successes of his forces have only hardened that view.
The HNC's "Vision for Syria" was unveiled in London by its general co-ordinator, Riyad Hijab, who defected in 2012 while serving as prime minister.
He said the plan envisaged three phases:
If the document on a transition in Syria had been produced by the political leaders of a victorious army at the gates of Damascus, it could be seen as an enlightened blueprint for the future.
But the regime in Damascus will dismiss it, because President Bashar al-Assad seems more secure in the capital than at any other time since the war started.
In 2013, when the chemical attack on the city's rebel-held eastern suburbs seemed certain to provoke American air strikes the regime looked shaky and afraid. But the strikes never came, and since then the president and his men have strengthened their positions. Crucially, Russia has intervened directly on the battlefield. President Assad cannot win as things stand, but he cannot lose either.
The war gets more complicated every year. Regional superpowers, and the world's most powerful countries have intervened. Dozens of different, and sometimes overlapping rebel groups operate on the battlefields. All have their own agendas. It's a mini world war. No wonder diplomacy fails.
Saudi Arabia's foreign minister said the plan would test Mr Assad's allies.
But Adel al-Jubeir told the BBC he was not optimistic that Russia and Iran were prepared to put the "necessary pressure" on the president "in order to comply with the will of the international community".
Mr Jubeir will discuss the HNC's blueprint with other foreign ministers from the Friend of Syria group of countries that support the opposition.
UK Foreign Secretary Boris Johnson, who is hosting the meeting, wrote in The Times that there "was still a chance that this vision can be made to work".
"If the Russians and Americans can together create a ceasefire, then the talks can restart in Geneva with the difference, perhaps, that all sides will by then have seen at least the scaffolding of a post-Assad Syria," he added.
Moscow and Washington, which backs the opposition and insists Mr Assad must go, have so far failed to reach an agreement on a new cessation of hostilities.
After a meeting with President Vladimir Putin at the G20 summit in China on Monday, Barack Obama said they had been unable to close "gaps of trust".
US officials are particularly concerned by the deteriorating conditions in and around Syria's second city, Aleppo, where rebel-held eastern areas are once again under siege after government forces advanced over the weekend.
On Monday, more than a 100 people were admitted to hospital following a suspected chlorine gas attack on the rebel-held Sukkari district. Activists in the city said one person died after bombs were dropped from government helicopters.
The umbrella group representing Syria's political and armed opposition factions has set out a plan for a political transition to end five years of war.