Glossary of maritime terminology

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OBO Ship

Short for ore-bulk-oil in freight terms, the OBO ship or OBO carrier is a multipurpose vessel which transports ore, heavy dry bulk goods, and oil.


An Officer is any licensed member of the ship's crew. A licensed mariner is a seafarer who holds a license from a maritime authority enabling them to hold senior officer-level positions aboard a vessel.

Offshore Barge

Like any barge, an offshore or oil barge is a wide flat bottomed boat which is not self-propelled and is tasked with carrying a cargo - in this case, oil. However, unlike an oil tanker, which is used to carry huge quantities of oil over long distances, offshore barges are smaller and designed for short-distance transportation.

Offshore Construction Vessels

Offshore construction vessels assist in the construction of various structures at sea and vessels include:

Offshore Production Vessels

Offshore production vessels assist in the drilling production processes and this category of offshore vessel includes:

  • Floating production storage and offloading vessels (FPSO vessels)
  • Single point anchor reservoir platforms (SPAR platforms)
  • Shuttle tankers
  • Tension leg platforms (TLPs)
  • Extended tension leg platforms (ETLPs)

Offshore Support Vessels

Offshore support vessels support the oil exploration and drilling vessels and these OSVs include:

  • Platform supply vessels (PSV)
  • Anchor handling tug supply vessels (AHTS)
  • Accommodation vessels (ACV)
  • Seismic survey vessels

Offshore Vessels

Offshore vessels are ships that are used for the exploration, drilling, and production of gas and oil in offshore environments. Offshore vessels can be split into four main types:

  1. Oil exploration and drilling vessels 
  2. Offshore production vessels
  3. Offshore support vessels
  4. Offshore construction vessels

Oil Exploration & Drilling Vessels

Oil exploration and drilling vessels are used to find and drill for oil and gas at sea and this category of offshore vessel includes:

  • Drillships
  • Semi-submersible vessels
  • Jack-up rigs
  • Offshore barges

Oil Purifier

In maritime terms, the oil purifier on a ship is used to separate water and other liquid and solid contaminants from the oil used as fuel. The majority of liquid contaminants are salt- and freshwater while solid contaminants are usually sand, rust and dust. Oil purifiers use vacuum dehydration technology and are an essential piece of machinery as they help to maintain the performance of the turbine and reduce the effects of wear and corrosion on the engine.

Oil Record Book

The Oil Record Book is a book or log kept by the Master of an oil tanker in which every discharge or leakage or spillage of oil must be recorded.

Oil Tanker

An oil tanker is a vessel that has been designed for the transportation of oil in bulk. An oil tanker’s cargo space consists of several or many tanks and tankers load their cargo by gravity from the shore or by shore pumps and discharge it using their own onboard pumps.


In maritime terminology, an Oiler is an unlicensed crew member who works in the engine room/department in this entry level cargo ship job. The Oiler greases and oils bearings and moving parts of the main engine and auxiliaries. However, the majority of these tasks are now done automatically and it is the oiler’s responsibility to ensure that everything is running correctly.

Oily Water Separator

An oily water separator, also known as a bilge water separator is a piece of marine equipment that is used to separate oil and water. It separates oil from oily water so that only water is pumped into the ocean when bilge water is being discharged or when oil tanks are being cleaned.

Online Maritime Directory

An online maritime directory is a place where companies who work in the shipping industry can add their business's details, contact information and 'about us' text in order to attract potential clients who may be seeking their services. 

For example, in the crewing sector, a Crew Manager may be looking to connect with a new manning agency or needing to find a new seafarer training provider. Online maritime directories make it easy for anyone seeking crewing services to find who or what they're looking for.

Open Registry

Also known as ‘Flag of Necessity’ or ‘Flag of Convenience’ in shipping terms, an Open Registry refers to flagship registry systems in some countries that allow vessel owners to register their ship under the flag of that country. This is normally done so that the shipowner can take advantage of favorable laws or rates that their own country does not have.

Ordinary Seaman

An Ordinary Seaman is a member of the Deck Department. They are a rank lower than an Able Seaman but will still be expected to know the ins and outs of their job. An OS’s duties and responsibilities include cleaning and maintenance, supervising lower ranking deck crew, and assisting with operations such as docking, undocking, line handling and the loading and unloading of cargo.

Ore Carrier

An ore carrier is a vessel designed for the transportation of ore. Ore has a high density therefore ore carriers have a relatively high center of gravity to prevent them from rolling in heavy seas.

Ore-Oil Carrier

Similar to an OBO ship, which carries ore, bulk goods and oil, an ore-oil carrier or oil-bulk carrier transports either oil or ore in bulk.


Oscar stands for the letter O in the International Radiotelephony Spelling Alphabet, which is most often referred to as the NATO Phonetic Alphabet or simply the Phonetic Alphabet. This is the most commonly used group of code words used to clearly communicate the letters of the Roman alphabet, particularly over the radio and is essential in helping seafarers give and receive messages, orders and instructions clearly and correctly.


OSV stands for either Offshore Support Vessel or Offshore Supply Vessel. The vessels that fall under these two categories are generally used in the oil and gas industries and will act as support to ships, platforms and rigs that are exploring, drilling or producing these commodities.

Some examples of offshore support and/or supply vessels are accommodation ships, anchor handling tug supply vessels and platform supply vessels.

Outfall Buoy

An outfall buoy, or outfall mark, is a buoy that marks where a drain or sewer discharges into the sea. 


Overtonnaging is a term used to refer to a situation when there are too many ships overall, or in a particular area or sector, for the amount of available cargo.