Here's what Julian Schnabel, the director of Before Night Falls, had to say about Olivier in an article that you can find here.
**Note from me: According to other articles and interviews, Olivier's father, while of Spanish heritage, was born in Morocco and Olivier's mother is from Brittany, France (which, however, wouldn't preclude the possibility that she is of Arab descent). Oh well... it's kind of difficult to get a handle on the background of OM's parents because each article seems to say something slightly different! If it really interests you, you can find references to OM's parents here (Brother Vincent talks about his father), here (Translation of Italian Vogue interview), and here (W Magazine interview). You'll see what I mean about conflicting information, but I'd be inclined to believe what seems to have come directly from Olivier's (and Vincent's) mouth in direct quotes, in which case his father was born in Morocco and his mother in France.