O.J. Simpson has a lot of time on his hands and he's putting it to good use -- he's in the best shape of his post-murder football life.

The Juice is dropping the lbs, lifting weights, and, for good measure, preaching racial harmony and anger management to other inmates at Nevada's Lovelock Correctional Center, a good friend of his tells the NY Post.

Norman Pardo claims, "He says that's one of the benefits of not being able to go out and party all the time like he used to."

Because idle hands are the devil's workshop.