A separator separates water and oil. An outer housing has an inner volume and an inner housing. One port in the inner housing receives influent mixed liquids. Another port from the inner housing discharges water outside the outer housing and defines a static liquid level. An outlet above the static level from the inner housing to the inner volume allows oil held in the inner housing to flow to the outlet. Surges of influent temporarily raise liquid levels, so oil passes the outlet to the inner volume. A supplemental separator in the outer housing removes oil from the inner volume.