An ore-bulk-oil carrier, also known as combination carrier or OBO, is a ship designed to be capable of carrying wet or dry cargoes. The idea is to reduce the number of empty (ballast) voyages, in which large ships only carry a cargo one way and return empty for another. These are a feature of the larger bulk trades (e.g. crude oil from the Middle East, iron ore and coal from Australia, South Africa and Brazil). (source)