Mel Gibson Under Oath

10/26/2010 9:00 AM PDT

Mel Gibson Under Oath

Mel Gibson will -- for the first time -- swear to tell the truth, the whole truth, and nothing but the truth about Oksana Grigorieva and their custody war ... and we've learned it's all happening Monday.

Our sources say Mel will show up at a "neutral location" Monday for his deposition.  But here's the deal.  The whole alleged domestic violence thing is off-limits, per the judge.  The subjects are custody and child support -- period.

Sources tell TMZ Oksana is scheduled to be deposed this Friday  -- but we're told her lawyers will go to court this AM and ask that Mel go first ... since Oksana has already sat through three depo sessions. Our sources say the judge isn't likely to go for it.

As for Oksana, the extortion claims will be off-limits as well.