Oksana's Former Bodyguard -- Now Protecting Herman Cain

Kristian Otto Herzog -- the guy who was once at the center of the bitter battle between Oksana Grigorieva and Mel Gibson -- is now at the center of the race for the Republican presidential nomination ... alongside Herman Cain.

Herzog was spotted on Friday with the GOP candidate during a campaign stop in Detroit, MI.

Cain's spokesperson, J.D. Gordon, tells TMZ this was Herzog's first job with the campaign and he worked with a team of 2-3 guys. Our sources say they don't believe Herzog is scheduled for any future events ... nor do they think he will be asked to provide future services.

Herzog worked as Oksana's bodyguard and claimed to have had a sexual relationship with her. He sided with Mel when she accused him of domestic violence.