When asked about his four children during an appearance on NBC's "Today" show, the 37-year-old actor said there was "another on the way."
"We're just starting to figure out how it works," he said Wednesday. "I think I know why this is happening, but we're gonna figure it out. Very excited, though. We're really excited about it."
The couple, who were married in 1997, have three boys and a girl. Fentress gave birth to their fourth child last year.
O'Donnell's screen credits include "Batman Forever" and "Scent of a Woman." He stars in the miniseries "The Company," which premieres Sunday on TNT.