Mel's rep confirmed the news to People, which reported the story on Monday. The confirmation comes after weeks of rumors in the media and on the Internet suggesting Oksana was pregnant.
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"He couldn't be happier," a friend of the actor told People. "He loves being a father."
Mel has seven children with wife Robyn Gibson. Mel and Robyn both filed for divorce last month after being separated for over three years.
According to the source, Oksana's pregnancy is unrelated to the divorce filing.
"This has nothing to do with the divorce. Mel didn't even know Oksana was pregnant when they filed," the source told People.
Mel and Oksana, 40, made their first public appearance together at the Los Angeles "X-Men: Wolverine" premiere in late April.
A source close to the actor previously told Access Hollywood that Mel and Oksana have been together less than one year.
Oksana has a son, Alexander, with her ex, former Bond star Timothy Dalton.