The person who knows more about Oksana Grigorieva's claims against Mel Gibson than anyone tells TMZ ... Oksana is grossly exaggerating or downright lying about the alleged domestic violence incident.
Kristian Herzog -- who served as Oksana's bodyguard until recently -- tells TMZ, Oksana admitted to him Mel never punched her, as she claimed to authorities. Rather, Herzog says Oksana told him Mel slapped her, and that's it. As TMZ first reported, Mel admitted slapping Oksana because he says she was shaking baby Lucia wildly.
Herzog -- who has been subpoenaed by Mel's lawyers in the custody war -- tells us Oksana admitted to him that she secretly taped Mel Gibson "to pressure Mel to pay her money." Oksana is currently being investigated by the L.A. County Sheriff's Department for alleged extortion.
Herzog -- who was privy to virtually every conversation Oksana had with her lawyers -- claims he had a sexual relationship with Oksana -- she denies it.