Up to 200 people, most of them aged between 20 and 30, attended the weekend party in an abandoned bunker in central Oslo, authorities said.
Private gatherings of more than 20 people are currently prohibited in Norway because of the new coronavirus pandemic, while public gatherings are limited to 200 people under strict conditions.
Police discovered the illegal rave when a patrol came across a group of distraught youths late Saturday in a central Oslo neighbourhood.
Officers then found several people unconscious in a former public safety bunker.
A total of 27 people, including two police officers who entered the site, were hospitalised. Five were in serious condition but out of danger.
They were all believed to have suffered from carbon monoxide poisoning caused by portable generators.
Five were released from hospital on Sunday, and several others were due to be released Monday.
Three remained under observation or in intensive care, the Oslo hospital said.
Police said they had an opened an investigation.
“Two people have been charged in this case, for trespassing and unauthorised use of the bunker,” a statement said.
“Police will examine whether more people will need to be charged in this case or if the charges need to be widened,” it added.
No arrests have been announced at this stage.
According to emergency services, the consequences could have been catastrophic. Invisible and odourless, carbon monoxide can be deadly in high doses.