The outbreak took place at a Christmas party on Nov 26 organised by renewable energy company Scatec, which has operations in South Africa where the variant was first detected.
"This party has been a super spreader event," Preben Aavitsland, a senior physician at the Norwegian Institute of Public Health, told Reuters by email.
"Our working hypothesis is that at least half of the 120 participants were infected with the omicron variant during the party. This makes this, for now, the largest omicron outbreak outside South Africa."
Aside from the people infected at the party, Aavitsland said, two people living on the country's west coast and two people in quarantine at Oslo airport have been confirmed as carrying the omicron variant of the coronavirus.
The outbreak led the Norwegian government to reintroduce some nationwide restrictions to curb the spread of COVID-19.
Across Europe, companies are cancelling Christmas party plans due to the emergence of the omicron variant.
The first person in Oslo confirmed as infected had attended the party, where at least one employee had just returned from South Africa. All the attendees were fully vaccinated and had tested negative before the event.
"Health authorities have confirmed a further 12 cases of omicron in Oslo after an outbreak," the city of Oslo said in a statement. "So far 13 omicron cases have been confirmed after sequencing. More cases are expected."
Health authorities said the individuals infected were so far displaying mild symptoms, with none hospitalised.
"It is still too early to say whether the clinical picture of the disease is different in omicron infections than in Delta infections," Aavitsland said.
"None of the patients has severe symptoms; none is hospitalised. However, this is not unexpected given the young age of the participants."
Scatec said it had been following authorities' advice throughout the pandemic. It also said its focus was on taking care of its employees and on limiting the spread of the virus.