Abstract: The inventive oil separator provided for de-oiling a gas exhausted from a crankshaft case (1) comprises oil collecting means (11) associated with a siphon (12) whose inlet inside the oil separator (7) is encompassed by an oil recovery chamber (13) provided with a perforated wall, at least a small oil recovery hole (14) arranged in the lowest part of the chamber (13) at the exit of the collecting means (11) and at least one depressurising small hole (15) which is arranged in the highest part of the chamber (13) above a clarified oil surface and is connected to the chamber (13) at a depressurising point (16). Said invention is usable for internal combustion engines.