Combination Carrier (Oil/Bulk/Ore, OBO)

A combination carrier able to load either liquid oil or dry bulk (ore, grain) in the same holds, switching between trades to cut ballast voyages; OBO denotes the oil/bulk/ore configuration, OO the oil/ore variant. The design avoided one-way empty legs (oil out, ore back) but needed expensive hold cleaning between cargoes and faced corrosion and structural fatigue from mixed loading. MARPOL Annex I Reg.1 treats them as oil tankers when carrying oil. The type is now rare; few new combination carriers have been built since the 1990s.