This week, an imposing room in an old mansion in central Oslo will come briefly to life for selection of this year's winner of the prestigious, sometimes contentious Nobel Peace Prize. The annual guessing game, now in full swing, rates Poland's Lech Walesa a leading nominee.

When the four men and one woman of the Norwegian Nobel Committee, along with the committee secretary, take their places around a massive oval table under a glass chandelier, a record 86 nominees will already have been winnowed to two or three finalists. This last of about six committee meetings in the room on the second floor of the Norwegian Nobel Institute will decide the winner, to be announced Oct. 14.

The deliberations of the five elderly Norwegians, chosen by the parliament here as decreed in the will of Alfred Nobel, are a closely guarded secret. The names of the nominees are not even disclosed unless made public by those who nominated them.

Among the known nominees this year are Walesa, leader of the Polish trade union Solidarity, and Lord Carrington, the British Foreign Secretary who negotiated the peace agreement that transformed the colony of Rhodesia into the independent African nation of Zimbabwe. Urho Kekkonen, president of Finland since 1956 and now 81 and ailing, has been a perennial nominee in recent years.

The committee usually arrives at a consensus on the winner, or on a decision not to award a prize, said the committee's secretary, Jacob Sverdrup. But even when there are disagreements, they normally are not made public. An exception was when two committee members resigned in protest when the other three were adamant about awarding the 1973 peace prize to then U.S. secretary of state Henry Kissinger and North Vietnamese envoy Le Duc Tho for the agreement ending the war in South Vietnam.

"Seldom is there strong disagreement," Sverdrup said. "Difficulties arise only when a member says, 'Under no circumstances can I accept it.' Then there has to be a vote. The only recourse after that is to leave the committee. No member can go out and say, 'I disagreed,' and stay on the committee."

Except for the chairman's formal statement explaining why the winner was chosen, made at the award ceremony in Oslo on Dec. 10, the committee does not answer criticism that has accompanied almost every prize selection in recent years. Among controversial winners besides Kissinger were two women in British-ruled Northern Ireland, Betty Williams and Mairead Corrigan, who founded a peace movement there that has since largely disappeared, and Israeli Prime Minister Menachem Begin and Egyptian President Anwar Sadat, whose Camp David peace agreement remains partially unfulfilled and strongly opposed in the rest of the Middle East.

"The committee's decision cannot fairly be judged historically," argued a source close to the committee, who said its mandate is to reward activities during the previous year to promote peace and brotherhood. "The committee cannot ask itself whether the Mideast treaty or the peace movement in Northern Ireland will last," he said. "The women in Northern Ireland did make a brave attempt to create peace there. It is unjust to say they didn't achieve very much. That is possible only in hindsight."

An important consideration in awarding the prize in recent years also has been to encourage the peace process or to promote human rights, this source said, pointing to prizes given to Williams and Corrigan in 1974, Soviet dissident Andrei Sakharov in 1975, the London-based human rights group Amnesty International in 1977, and Argentine dissident Adolfo Perez Esquivel in 1980. The source noted that both Albert Schweitzer, awarded the prize for 1952, and Willy Brandt, who won it in 1971, said they felt obligated by the honor to devote the rest of their lives to the cause of peace.

But the committee's secrecy has nurtured frequent speculation about whether it is influenced by politics or campaigns mounted on behalf of some nominees. Chosen by the Norwegian parliament for six-year terms, which are frequently renewed, the committee's members currently come from four different political parties and professions ranging from history professor to Norway's official film censor. The chairman, John Sanness, 68, founded the Norwegian Institute of International Affairs.

The Norwegian Nobel Committee is one of six that select annual winners of the prizes established in the will of Nobel, the Swedish inventor of dynamite and other explosives. The other five, which award the prizes for physics, chemistry, medicine, economics and literature, are based in Stockholm. No one knows why Nobel decided that Norwegians should administer the peace prize.

The Norwegian committee solicits nominations each year from previous Nobel winners, present and former committee members, several international institutions and parliamentarians and academics around the world. Only living people or existing organizations like the International Red Cross, which has won twice, can be awarded the prize. This year's winner will receive $175,000, a gold medal and a certificate.

With the help of its secretary and a small circle of consultants among Norwegian academics, the committee culls the nominees to about 10 serious contenders each year, Sverdrup said, and then chooses the two or three finalists. Their merits are debated extensively, he said, before the selection is made.