Oksana recorded the conversations without Mel's knowledge. In California, if a person isn't told their conversation is being recorded, it is not admissible in court.
There are exceptions -- notably, if Oksana reasonably believed the conversation related to the commission of any felony of violence, she could make the recording without Mel's permission.
Here's the problem. TMZ has learned Oksana sent Mel several text messages in which she clearly states she recorded the conversations because Mel didn't keep his part of the deal to support her.
We're told Mel's people will use Oksana's text messages to try and prove it was just a money grab, which would make the recordings inadmissible in both the custody case and any possible criminal case.
Sources connected with Oksana say ... if she was motivated by money she would not have rejected Mel's $15 million settlement.