The actress, 27, has been dating the French banker, 44, for nearly two years.
US Weekly reports that the pair have made their relationship more official after she accepted his recent proposal.
The Weeds star was first seen publicly with the half-brother of former French president Nicolas Sarkozy in May 2012. And, despite the age gap (and huge height difference!), the pair have been dating ever since and live together in New York.
If the reports are true, this will be Olivier's second marriage and Mary-Kate's first. He has two children, Julien, 12, and Margo, 10.
Actors Mary-Kate and twin Ashley have been in showbiz since they were six. Their younger sister Elizabeth has also had acting success.
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