"An event cancelled is better than a life cancelled" said WHO head Dr Tedros Adhanom Ghebreyesus, adding that "difficult decisions" must be made.
"In some cases, that will mean cancelling or delaying events" he said.
He added that there was now "consistent evidence that Omicron is spreading significantly faster than... Delta".
Dr Tedros' comments come as a number of countries - including France and Germany - have tightened Covid restrictions and imposed travel curbs to try to halt the spread of the new variant. The Netherlands has introduced a strict lockdown over the Christmas period.
The White House said on Monday that President Joe Biden was not planning on "locking the country down". The country's top infectious disease expert, Dr Anthony Fauci, earlier warned that Christmas travel would increase the spread of Omicron even among the fully vaccinated.
In the UK, Prime Minister Boris Johnson said on Monday that the government needed to "reserve the possibility" of bringing in new rules in England as Omicron cases surged, but did not announce further restrictions.
New Year's Eve celebrations in London's Trafalgar Square have been cancelled "in the interests of public safety", Mayor Sadiq Khan said.
Omicron - first detected in South Africa in November - has been classed as a "variant of concern" by the WHO.
Photo BBC screenshot Caption: WHO head Dr Tedros Adhanom Ghebreyesus