Correction to all.
An oil/air separator simply allows your engine crankcase to "breathe" and separate oil from air before returning it to the intake.
With the parts moving inside the engine (pistons up and down) there can be a marginal amount of air displacement.
This air has to come and go from somewhere.... in standard form, there is a provision made, but can feed oily air back in to the intake.
By fitting an oil/air separator, it allows the oil to be removed from the air before returning back to the intake.
It also reduces crankcase compression that is caused by the displacement of air and blowby caused by worn pistons/rings