The number is likely to grow as sequencing is carried out on other positive tests from party-goers.
“So far 60 people have tested positive (for Covid) with PCR tests, and four with antigen tests,” the city of Oslo said in a statement.
“Seventeen are probably Omicron, but that has yet to be confirmed. So far, one case is confirmed to be Omicron after sequencing,” it said.
Between 100 and 120 people — all of whom were vaccinated, including one who had recently travelled to southern Africa — had gathered last Friday for a Christmas party organised by their employer.
“All of them had been vaccinated, none of them had symptoms and they had all done self-tests” before attending the dinner, city health official Tine Ravlo told AFP.
“Everything was done in line with regulations and no rules were broken,” she said.
All of those who have tested positive have so far only had mild symptoms, such as headaches, sore throats and coughs, Ravlo said.
The Omicron suspicions arose after screening was conducted on some of the tests, which have not all been fully analysed through sequencing yet.
That means the number of confirmed cases could grow. Further results were expected sometime Friday.
On Thursday, after fears of the suspected cluster emerged, the Norwegian government announced a slew of restrictions in greater Oslo which entered into force at midnight.
Face masks are mandatory in public transport, shopping centres, shops and taxis when social distancing is not possible. People will have to work from home if possible and the number allowed to gather at private indoor events will be limited to 100.