Officials at Primarily Primates in San Antonio believe Oliver was at least 55 years old.
Caretakers found Oliver dead Saturday in his hammock. Sanctuary officials say the former circus performer was most blindly, had no teeth and suffered from arthritis.
Oliver had been at the Texas sanctuary since 1998, after being relocated from a research lab in Pennsylvania.
The sanctuary's website says Oliver was born in Africa and spent his early years displayed as the "Humanzee."
Experts at the University of Chicago who in 1996 did DNA studies on Oliver found no evidence of human chromosomes.
The chimp was the subject of a 2006 Discovery Channel documentary.