Just six months after giving birth to baby Otis, Olivia looked back to her pre-baby best, even flaunting her midriff in a beautiful slinky cutout Michael Kors Resort 2015 gown.
The dress also added extra glamour with a side panel of sequins and a back split, which revealed her amazing ankle-strap heels.
Olivia arrived at the bash on the arm of her beau Jason Sudeikis, who stars in the sequel to the 2011 hit comedy, which saw three friends hatch a plan to kill off their 'horrible bosses'.
Jason was also smart in a three-piece checked suit, but he looked happy to let Olivia lap up the limelight in her amazing frock.
As usual, Olivia's hair and makeup was flawless, as she worked 1970s-style middle-parted loose waves and liquid-lined eyes.
Other stars at the bash included Jason Bateman and Amy Childs, but it was Jennifer Aniston who stole the show in her incredible Antonio Berardi Spring/Summer 2015 LBD with a red insert at the bust.
There was no sign of fiance Justin Theroux, but as he jetted into London with the star, he was no doubt either waiting for her at the after-party or back at their (undoubtedly amazing) hotel room.
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