O.J. Simpson is concerned Bill Cosby will be attacked by some "nut" in prison -- and he would know, right?
Juice spent 9 years behind bars in Nevada for kidnapping and armed robbery and warns that people like Cosby -- convicted sex offenders -- don't do well behind bars.
"The problem is the nature of the crime," O.J. said while golfing out in Nevada ... "Rapists are frowned upon in prison."
"I'm sure they'll be guys protecting him but they're gonna have to put him in protective custody .