Oksana Under Oath ... Again

Oksana Grigorieva is about to raise her right hand for the fourth time and sit for a deposition ... TMZ has learned.

As we first reported ... Oksana's lawyers tried to delay this deposition until Mel sat for his ... which is scheduled for Monday.  The judge would have none of it, so Oksana must show up today.

The depo will cover custody issues, but we're told Oksana's statements to PEOPLE will also be hot topics.