"I'm so thrilled about having a baby boy and starting a new chapter as a parent with David," the 29-year-old actress told People.
"It's amazing how in love I am with this little person I've only seen on a black and white screen. I'm starting my own family and there is no other feeling like it," she added.
Ruah and Olsen -- the older brother of NCIS: Los Angeles star Eric Christian Olsen -- met on the set of the show. David Olsen is a former Navy SEAL who works as a stunt double on the CBS drama, which debuts its new season September 24.