The 29-year-old actress became engaged to the comedian in January and new reports suggest the pair are soon to start a family.
According to America's People magazine, representatives for both stars confirmed the baby news.
“They are incredibly happy,” a source said.
“They’re very excited to welcome a new member into their family.”
A spokesperson for the couple did not offer comment.
Olivia opened up earlier this month about parenthood but left no hints she was expecting.
Writing an article in British magazine Glamour about the do's and don'ts of turning 30, Olivia, who hits the landmark birthday next March, urged women not to rush into motherhood.
"Don't feel pressured to pop out kids. I love kids with a passion I usually reserve for hot cheese, miniature chairs and Prince concerts, but I feel no stress to reproduce simply because of a fear of withering eggs," she advised. "Wait for the right partner, and make sure you're where you want to be in life before picking your home based on school districts. This is not to suggest you should live irresponsibly for the next ten years, then expect to get knocked up."
Olivia and Jason met on the set of US comedy sketch show Saturday Night Live in 2011.