EXCLUSIVE

Oksana Grigorieva -- the woman who Mel Gibson struck in the mouth -- just lost hundreds of thousands of bucks because of her big mouth ... by violating her confidentiality agreement. ... court sources tell TMZ.

It all has to do with an interview she did in October with Howard Stern. Oksana -- who promised in her settlement agreement with Mel she would say NOTHING about him publicly -- blabbed about the actor.

In the Stern Interview:

-- Howard sympathizes about her problems with Mel, telling her she has to go forward in life. She refers to her "painful and dark" experience and says now she wants to help others.

-- Howard asks if Mel wined and dined her. She says it's too painful to even talk about.

-- Howard talks about how amazing the famous audio tapes are, and Oksana responds, "Life happens ... I'm still catching up on the humor part."

-- Howard, who referenced portions of Mel's rants during the interview, told her a woman shouldn't be treated that way, especially the mother of your child. She said, "Thank you."

Apparently that was enough for a judge, because court sources tell us, hizzoner ruled Oksana was never supposed to talk about Mel -- even obtusely -- and as a result she forfeited the remaining installments of her $750,000 settlement. We're told she was still owed half -- $375,000 ... money she now will NOT get.

TMZ broke the story ... Oksana has filed for bankruptcy. Pretty amazing, since Mel initially offered her $15 MILLION but she rejected it.