Emergency services found seven people unconscious in the bunker in the city’s St Hanshaugen area.
The lives of all of the victims, believed to be mainly aged between 20 and 30, are said to be out of danger but police have called for other attendees to seek medical attention if they start suffering from nausea, headaches or dizziness.
Carbon monoxide is an odourless, colourless gas produced by incomplete burning of carbon-based fuels, including gas, oil, wood and coal. The gas is harmful because it displaces oxygen from red blood cells, resulting in damage to major organs.
One man who left before police arrived told public broadcaster NRK he had gone outside repeatedly for fresh air after arriving for the rave at 23:00 (21:00 GMT) on Saturday.
“When you rave in a room without windows with 50 to 60 others, the air becomes heavy,” the unnamed man said. “I had to go out several times and breathe fresh air.”
The same man said other parties had been held in the same bunker this summer.
Generators were being used to power lighting and sound systems, Norwegian media report.
Vidar Haukeland, head of the company which owns the bunker, Stiftelsen Diakonissehuset Lovisenberg, described the rave as a “serious break-in” and said the company did not feel any responsibility, VG newspaper reports.
Boards covering the entrance to the bunker had been removed, the paper says.
Earlier this month, Norway banned the serving of alcohol in bars after midnight until further notice after a rise in coronavirus infections.