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, designed this Black & White 1/2 Skull Neoprene Face Mask with other inmates at Nevada's Lovelock Correctional Center, a good friend of his tells the NY Post.

Jack Criswell claims, "He says that's one of the benefits of not being able to go out and ride his motorcycle all the time like he used to."

Because idle hands are the devil's workshop.

Neoprene face mask features full coverage of the face and ears. Stretchy neoprene material is warm and water resistant. Patterns are reversible to solid black material. Velcro closure ensures great fit with goggles, with a helmet or without. One size fits most. Black and white skull design.