Around 200 people attended the illegal party in the bunker on Saturday night.
And there was only one way out, a narrow opening of about once a meter.
When the police arrived at the scene, seven unconscious people were found, and over 20 party participants were taken to hospital.
According to the police, the carbon monoxide poisoning must have been caused by units in the bunker that were used to supply power to, among other things, a music system.
Some are in intensive care
Dag Jacobsen, who is head of the emergency department at Oslo University Hospital, says that it was close to a national disaster.
- We have patients who are still hospitalized for treatment and who have complications in the form of brain damage, he says to NRK.
For reasons of confidentiality, he does not want to say how many are cared for at the hospital, but it is about "several people", some of whom receive intensive care.
"Not enough to survive"
Dag Jacobsen now wants to warn other young people who plan to go to similar rave parties.
- It is not good enough to survive a carbon monoxide poisoning.
If you are a teenager, do not live the rest of your life with brain damage.
Carbon monoxide is a very toxic gas, which does not smell, taste or feel otherwise.
It can occur in exhaust gases and fire gases, according to the Poison Information Center.
Symptoms of poisoning may include headache, dizziness, fatigue, nausea, vomiting and unconsciousness.