 If the oil content is higher than 15 ppm, the OCM will activate an alarm and move a three-way valve that, within a short period of time, will recirculate the overboard discharge water to a tank on the OWS suction side.An OCM takes a trickle sample from the OWS overboard discharge line and shines a light through the sample to an optical sensor.Since small oil droplets will diffract and diffuse light, a change in signal at the sensor will indicate the presence of oil.