Oksana's Former Bodyguard
Now Protecting Herman Cain
 Herman Cain bodyguard
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 -- a convicted felon with a penchant for impersonating police officers -- 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.

Kristian Otto Herzog