According to Us Weekly, the Australian actor and his girlfriend Malia Jones will be welcoming their first child together in the coming months.
This will be O'Loughlin's second child. He already has one son, Saxon, from a previous relationship.
O'Loughlin is best known for his roles on Hawaii Five-0 and as Jennifer Lopez's love interest in The Back-up Plan.
The 35-year-old actor recently took a break from acting for a brief stint in rehab in order to receive "supervised treatment for prescription pain medication".
O'Loughlin will only be missing from one episode in the upcoming season of the show, which returns on Monday, September 24 at 10/9c on CBS.
Watch a promo for the Hawaii Five-0 season two finale below: