The air oil separator is an important part of your air compressor. Basically, it is responsible for separating the air from the oil. You use compression oil to lubricate and to cool the device during the process of compressing air. There should be no traces of oil in the compressed air if the oil separator is working, as it should. The oil separator sees to it that the oil will leave the compression chamber and return it to the main oil sump to be used again. The oil separator is one of the air compressor parts that need regular maintenance to ensure that your machine works efficiently.