The 25-year-old has been dating Olivier, half-brother of former French President Nicolas Sarkozy, for around four months and now the couple are set to spend a lot more time together, reported New York Post online.
Olivier, 42, is buying a townhouse here and although they won't officially be living together, Olsen will be there most of the time.
"They are really, really happy. And Olivier is buying a townhouse below 14th Street, to be with Mary-Kate. We are told they're not officially moving in together - they both think it's too early for that. But he's telling friends that Mary-Kate is the best thing that has happened to him in a long time," a source said.
Olsen previously dated artist Nate Lowman for over two years before splitting in 2010, while Olivier romanced actress Stella Schnabel for a year before their break-up in March.
Olivier has two children with former wife Charlotte, who has called his relationship with Olsen "grotesque" and "not right".