Coming out the bottom of the AOS is a 90 degree curved elbow that is the drain port. This is where the separated oil exits and runs back to the oil pan. This picture shows the 1-way check valve that came with the AOS drain kit so when there is positive pressure in the oiling system, it closes off and doesn't let that back feed into the AOS. When the car is turned off, that oil will use gravity to drain the line back to the oil pan.