Always thought it was his son (with Marguerite not Nicole), Jason. Dude was a hand-to-hand and weapons combat expert in the Army. 6'0" 230 lbs and was in his late 20's. Him taking out Nicole and Ron Goldman always made sense. OJ had 2 arthritic knees, was 47 years old yet somehow took out Ron (super athletic, martial arts black belt) Goldman with nary a scratch. OJ winning is one thing, but there's no way he could have done it that clean by himself.
I don't think Jason even took the stand. Jason was also pissed off with Nicole because he was the chef at some restaurant and wanted her to come by but she didn't..hanging out with Ron, I think. My guess is that OJ/Jason were both there and Jason did most of the work.
OJ was also still upset because Marcus Allen had been smashing Nicole. Dude's nickname was 'Driftwood' because he apparently was #TeamEggplant. Rumor has it that his beef with Al Davis stemmed from Marcus Allen smashing half the wives of coaches on the team.