As the annual Nobel prize-giving week reaches its peak, the five-member Norwegian committee will unveil its decision at 0900 GMT, the only one of six awards to be presented in Oslo and the one which traditionally garners the greatest attention.
For once, experts, online betting sites and commentators had thought they were on to a sure thing with Colombian President Juan Manuel Santos and FARC chief Rodrigo London, alias Timoleon "Timochenko" Jimenez, widely seen as frontrunners after signing a deal to end 52 years of civil war.
But they were suddenly forced to rethink after voters in Colombia rejected the agreement between their government and the communist FARC rebels in an October 2 referendum.
That threw the prestigious prize wide open again, and with a record 376 nominations to consider, predicting the winner is largely a lottery, with experts far from unanimous over who the committee will choose.
On the eve of the award, several Nobel watchers flagged civilian-led endeavours, with two betting sites giving Greek islanders the best odds for coming to the aid of thousands of desperate refugees landing on their shores after making the perilous journey across the sea from Turkey.
For others, it was the work of Congolese doctor Denis Mukwege in helping women recover from the violence and trauma of sexual abuse and rape in war-torn eastern Democratic Republic of Congo.
- 'Resilience and bravery' -
Syria's White Helmet volunteers were also touted as possible winners for their daring efforts to rescue civilians caught up in the carnage of the country's five-year war.
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Working in rebel-held areas, the force has won international plaudits for the bravery of its nearly 3,000 volunteers who risk life and limb to pull survivors from the rubble, with their nomination for the prize firmly backed by Britain's Guardian newspaper.
"What the White Helmets accomplish may seem like a drop in the ocean, but what they represent is immense: resilience and bravery in the face of barbarism," said the paper in an editorial.
"And they show that individual acts of courage can go a long way to fight indifference. They also embody a spirit of civic resistance... exemplifying courage and solidarity in the face of state-sponsored terror."
For Kristian Berg Harpviken, director of Oslo's Peace Research Institute (PRIO), the top contender was Russian rights activist Svetlana Gannushkina for her decades-long work with refugees and migrants -- an issue which has shot to prominence in Europe since the start of the migrant crisis.
Also in the running is Nadia Murad, a Yazidi who endured months of sexual abuse by Islamic State militants before escaping to become a global spokesperson for her people.
And US intelligence whistleblower Edward Snowden has also been touted for his exposure of the scope of US surveillance.
- Nuclear diplomacy -
If diplomatic achievement wins the prize, it could go to the negotiators behind the 2015 Iranian nuclear accord which effectively curbed Tehran's nuclear drive, putting an atomic bomb out of reach, in exchange for a gradual lifting of the crippling sanctions imposed on its economy since 2006.
That could see the prize going to Washington's top diplomat John Kerry, his Iranian counterpart Javad Zarif and EU foreign policy chief Federica Mogherini, as well as to nuclear experts Ernest Moniz, the US Energy Secretary, and Ali Akbar Salehi, head of Iran's Atomic Energy Organization.
In an illustration of just how difficult it is to call, last year's prize went to four Tunisian groups who were instrumental in the country's transition to democracy -- none of whom had been mentioned in any of the pre-announcement speculation.