An oiler's duties consist mainly of keeping machinery lubricated. As a member of the engineering department, the oiler operates and maintains the propulsion and other systems onboard the vessel. Oilers also deal with the "hotel" facilities onboard, notably the sewage, lighting, air conditioning and water systems. They assist bulk fuel transfers and require training in firefighting and first aid. Moreover, oilers help facilitate operation of the ship's boats and other nautical tasks- especially with cargo loading/discharging gear and safety systems. However, the specific cargo discharge function remains the responsibility of deck officers and deck workers.
A person has to have a Merchant Mariner's Document issued by the United States Coast Guard in order to be employed as an oiler in the United States Merchant Marine. Because of international conventions and agreements, all oilers who sail internationally are similarly documented by their respective countries.