The couple had been dating for over a year, and have five-month-old daughter Lucia together. The pair are said to have mutually decided to end their romance, but are still on good terms.
"They just drifted apart," a source told People magazine. "They`re both working hard on their careers and trying to raise a sweet baby together. They`re still friends and they`ll both raise Lucia together."
Mel is currently shooting new movie How I Spent My Summer Vacation in Mexico, while Oksana is working on her music career.
The pair were first seen in public together in April last year, shortly after Mel`s wife of 28 years Robyn filed for divorce. The actor has seven children with Robyn, while Oksana has a son with her ex-partner Timothy Dalton.
Mel`s representative has not commented on the split claims.