Abstract: The design and construction of past air-oil separators included labyrinth type designs with a series of baffles which can be expensive and complicated to manufacture. The present invention utilizes an air-oil separator which allows the introduction of blow-by gas to high velocity fresh air at its boundary layer in order to separate oil and air from the blow-by gas. The air-oil separator has an annular housing with first and second compartments. The blow-by gas enters a constricted middle portion of the first compartment through a plurality of openings. Simultaneously, fresh air at a high velocity flows through the constricted middle portion. The blow-by gas is accelerated with the fresh air flow at the boundary layer of the fresh air. The fresh air flow and blow-by gas reach an annular opening in the second compartment which strips the boundary layer from the fresh air flow causing the blow-by gas to split off into an annular outer gallery at a high velocity.