Former House and The OC star Olivia confirmed their happy news at the weekend, thanking fans for their messages of congratulations.
She tweeted: "WOW. You guys are awesome. So kind. Thank you thank you thank you! PS. Babies eat mostly salsa right?"
Olivia met comedian/actor Jason during an appearance on US sketch show Saturday Night Live back in 2011.
They announced their engagement in January this year, after Jason popped the question over Christmas.
At the time Olivia tweeted: "Thanks for all the sweet congratulatory love, friends! And may I compliment your savvy use of that nifty engagement ring emoticon."
Olivia often gushes about her love for Jason in interviews, telling InStyle earlier this year she's found a "real partner" who will always have her back.
"I like having a real partner. It's the model I grew up with. It's part of why I was able to take so many chances in my life, because I felt like someone was watching my back. I could never imagine life without someone I respect, someone who offers me infinite possibilities."
Olivia was previously married to Tao Ruspoli for eight years (they divorced in 2011), while Jason has one ex-wife, screenwriter Kay Cannon. They split in 2010.
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