More than seven years ago, Michelle Williams became a single parent when her ex, Heath Ledger, passed away.
Olsen's most recent long-term relationship with was with the artist Nate Lowman, from who she split in 2010.
She was a close friend of the late actor Heath Ledger, and was the first person his masseuse rang upon finding the Batman star unconscious.
Mary-Kate, 25, previously dated artist Nate Lowman for over two years before splitting in 2010, while 42-year-old Olivier romanced actress Stella Schnabel for a year before their break-up in March.
Olivier was previously married to wife Charlotte and they have two children together.
The New York Post's Page Six was first to break the news on Wednesday morning (May 30, 2012), with a source telling the paper that the couple had been dating for just over a month.