Ex-NFL star Shawne Merriman says O.J. Simpson absolutely should get to use his Heisman vote now that he's outta the slammer ... telling TMZ Sports it all boils down to a basic formula.
On the field ... O.J. was a legend who (fun fact) won his Heisman award by the largest margin of victory EVER.
Off the field ... the former Chargers linebacker says it doesn't matter what Simpson did or didn't do -- 'cause he's voting on a football award!
Bonus: Shawne went to town on the NY Giants for their "HORRIBLE" decision to bench Eli Manning for a week -- and he made some pretty good points.