The 'Drinking Buddies' star and her fiancé Jason Sudeikis welcomed their first child, Otis Alexander Sudeikis, into the world a few weeks early, as she previously revealed she was due on May 4.
The 30-year-old beauty announced the happy news on her Twitter page, writing: "Ladies and gentlemen, Otis Alexander Sudeikis has LEFT the building! (I'm the building)."
She also shared a black and white image of herself cuddling the newborn in a blanket.
The brunette beauty, who got engaged to Jason in January 2013, recently gushed about the bond she had formed with her son while pregnant.
She said: "He's moving around in there and no one else can tell - it's a little secret friendship."
Olivia also spoke about embracing her body during her pregnancy.
She said: "I just thought, Oh I'm going to hide this forever. But I ended up getting kind of excited to show the bump, as a badge of pride. Like, I'm a woman! Look at me making a human! I am a goddess!"
The 'Rush' star, who was previously married to Italian prince Tao Ruspoli for eight years until 2011, also predicted that Jason would be a great father.
She said: "Jason is fantastic with children."