Corrections Canada officials notified families of his victims Wednesday that the 71-year-old Olson likely has days to live in a Montreal-area hospital, Postmedia News reported.
He was being held at a maximum security prison in Quebec and undergoing treatment for an undisclosed cancer, but the disease apparently metastasized, officials said.
Olson was arrested in British Columbia in August 1981 and struck a deal with the Royal Canadian Mounted Police and prosecutors. After his wife and son were paid $100,000, Olson led police to the sites where he dumped his 11 victims.
A court heard he lured the eight girls and three boys between the ages of 9 and 18 with an offer of a job, then sexually assaulted and killed them.
Once he was in prison, he made claims he had killed as many as 200 other people, the report said.
He applied for parole several times, but was denied each time.