Obo ship: a multipurpose ship that can carry ore, heavy dry bulk goods and oil. Although more expensive to build, they ultimately are more economical because they can make return journeys with cargo rather than empty as single-purpose ships often must.

Ocean waybill: a document, issued by a shipping line to a shipper which serves as a receipt for the goods and evidence of the contract carriage.

OCMI: officer in charge of marine inspection.

Off-hire clause: in a time charter, the owner is entitled to a limited time for his vessel to be off hire until such time as the vessel may be repaired or dry-docked.

Officer: any of the licensed members of the ship's complement. 

Off-load: discharge of  cargo from a ship.

Oiler: an unlicensed member of the engine room staff who oils and greases bearings and moving parts of the main engine and auxiliaries. Most of this work is now done automatically and the oiler merely insures it operates correctly.

Oil record book: a book or log kept by the master of an oil tanker wherein every discharge or escape of oil is recorded. 

Oil tanker: a ship designed for the carriage of oil in bulk, her cargo space consisting of several or many tanks. Tankers load their cargo by gravity from the shore or by shore pumps and discharge using their own pumps.

Open rates: pricing systems that are flexible and not subject to conference approval. Usually applied to products in which tramps are substituted for liners.

Open registry: a term used in place of "flag of convenience" or "flag of necessity" to denote registry in a country which offers favorable tax, regulatory, and other incentives to ship owners from other nations.

Open top container: a container fitted with a solid removable roof, or with a tarpaulin roof, so the container can be loaded or unloaded from the top.

Ordinary seaman: a deck crewmember who is subordinate to the able bodied seaman. 

Ore carrier: a large ship designed to be used for the carnage of ore. Because of the high density of ore, ore carriers have a relatively high center of gravity to prevent them being still when at sea, that is, rolling heavily with possible stress to the hull.

Ore-bulk-oil carrier: a large multi-purpose ship designed to carry cargoes wither of ore or other bulk commodities or oil so as to reduce the time the ship would be in ballast if restricted to one type of commodity. This type of ship is sometimes called bulk-oil carrier.

Ore-oil carrier: a ship designed to carry either ore or oil in bulk.

Overtonnaging: a situation where there are too many ships generally or in a particular trade for the level of available cargoes.