Olivia Wilde has become a mum - to a baby boy called Otis.

The Rush actress has been expecting her first child with fiance Jason Sudeikis.

Olivia posted a black-and-white close-up of Otis snuggling up against his mother on her Twitter page , with the caption: "Ladies and gentlemen, Otis Alexander Sudeikis has LEFT the building! (I'm the building)."

The child is the first for both for the Tron: Legacy actress and We're The Millers and Horrible Bosses star Jason, 38.

Olivia and Jason began dating in 2011 and got engaged in January 2013.

She recently said of their wedding: "It's certainly not something I feel pressured to do before we have the baby. I'm such a believer in marriage and I think of him as my husband anyway."