Abstract: A method of draining oil from a transmission housing, of an oil-injected screw compressor comprising a compressor housing containing rotor means and a transmission housing. A shaft of the compressor extends into the transmission housing from the compressor housing and the transmission housing is underslung relative to the compression housing. The first portion of the compressor intake stroke, or rotation, is prevented from being filled with intake gas such that a depression is created. Said depression is applied via a bore to a sump portion of the transmission housing, so that the absolute pressure differential between the sump and the suction portion of the rotor means draws oil out of the sump and into the rotor spaces. A compressor employing this method of draining oil is also described.