Olivia Wilde Shares Sweet Happy Birthday Tribute to 'Best Friend' Jason Sudeikis

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Cue the 'awww's!

Olivia Wilde posted a sweet happy birthday message to fiance Jason Sudeikis on Sunday, sharing an adorable throwback selfie for the occasion.

"Old pic of this old couple," she captioned the photo."Happy Birthday to my best friend, my favorite co-pilot on how-the-hell-do-you-raise-a-kid airlines, the greatest karaoke buddy known to man (or woman), the funniest, loveliest dude I've ever met, and much, much more. I love you."

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The Angry Birds actor turns 41 on Sunday.

Sudeikis and Wilde became engaged in January 2013, but have yet to tie the knot. The couple welcomed their first child, son Otis, in April 2014. They are currently expecting their second child together.

Sudeikis opened up to ET at the premiere of Angry Birds in May about his son's reaction to the new baby news.

"He knows that it's something in an egg," he said. "We just used metaphors in the movie [to explain it to him]. That's all. We explain everything through the movie."

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As it turns out, the 3-D animated comedy is particularly special for the Wilde-Sudeikis family.

"This is the first movie that we've sat through the whole time together," the actor said of his son. "He liked it a lot. I mean, he was riveted, which is pretty impressive to me. As I've told it to other parents, they were pretty shocked as well. But now he knows all the birds' names and everything. It's pretty adorable!"

See the interview below!

WATCH: Jason Sudeikis Opens Up About Otis' Reaction to Becoming a Big Brother