"Between eight and 10 have said they would come and and somewhat fewer have responded in the negative," the director of the institute, Geir Lundestad, told AFP.
After the unexpected awarding of the peace prize to a crisis-stricken EU on October 12th the bloc's President Herman Van Rompuy invited all the leaders of the 27 member states to attend the ceremony, which is traditionally held on December 10th, the anniversary of Alfred Nobel's death.
The Nobel Institute refused to reveal the names of those who have accepted and declined their invitations, saying that the list changed "from day to day".
While German Chancellor Angela Merkel has announced that she intends to go to Oslo, her British counterpart David Cameron, facing pressure from the eurosceptic wing of his Conservative Party, wryly noted that "there will be enough people to collect the prize".
Cameron's decision to not travel to Oslo has been widely interpreted as a demonstration of his lack of enthusiasm for the EU.
France numbers among the countries that have yet to respond to the invite, according to the Nobel Institute. President Francois Hollande said last month he would also be in Oslo if all the other heads of state and government were to attend.
The Netherlands also said Wednesday it had not yet taken a decision.
"It would have been unrealistic to expect everyone to come," Lundestad said when asked about the lack of a unified response to the invitation.
The award, which consists of a diploma, a gold medal and a cheque for 8 million kroner ($1.18 million) will be formally handed over to Van Rompuy together with the presidents of the European Commission, Jose Manuel Barroso, and of the European Parliament, Martin Schulz.
Other European leaders will be seated in the audience.
The EU will donate the prize money to children affected by war and conflicts, officials said on Wednesday.