Oksana's Closest Advisor -- Long Rap Sheet

Kristian Otto Herzog has been working as Oksana's bodyguard for months, but multiple sources in the case say he has become so involved in the custody war, he is calling the shots. Herzog is the reason Oksana has changed lawyers so frequently -- when he doesn't like them, they go.

Herzog -- who now has what is described as a "deep relationship" with Oksana -- is constantly around 11-month-old Lucia, and Gibson is worried.

For starters, Herzog has repeatedly impersonated a police officer with loaded firearms.

In 2009 alone, he was arrested twice and each case involved firearms. According to court records, in April, Herzog chased two men who were driving in Hollywood, confronted them and accused them of cutting him off. Authorities say Herzog impersonated a cop in his Crown Victoria, with a placard on the dash which read "Police Interceptor." Herzog was carrying a loaded semiautomatic firearm.

In May 2009, Herzog followed a woman out of a Ralph's supermarket, turned on lights and a siren he rigged on his car ... and after she stopped, he began asking her personal questions. She freaked out, drove away and found a nearby cop, who arrested Herzog. At the time of his arrest, Herzog had a knife in his boot as well as a loaded semiautomatic handgun which was not registered to him.

Herzog was charged with multiple crimes and pled no contest in October, 2009 to two counts of impersonating an officer -- both felonies. He was sentenced to one year in jail but was given credit for time served -- he was in jail for 255 days after his second arrest in May. He is on three years probation and prohibited from possessing a firearm.

But recently, two people accused Herzog of making death threats against them for not testifying against Mel Gibson in the ongoing war.

Herzog, who has an explosive temper and uses it frequently with the players in the current case, including Oksana's lawyers, has been convicted of two other crimes.

It's ironic -- since their mediation, Oksana has claimed Mel Gibson is a danger to his children, yet she is allowing a man with a history of firearms abuse and a nuclear temper to essentially live at her home, where he is constantly around the baby.

Sources tell us ... Mel's people have asked Oksana to keep Herzog away from the baby, but she refuses.