An ordinary seaman is an unlicensed member

Licensed mariner

A licensed mariner is a person who holds a license issued by one or more countries to hold senior positions aboard ships, boats, and similar vessels. The United States Coast Guard grants licenses to members of the United States Merchant Marine in five categories: deck officers, engineers, staff...

 of the deck department

Deck department

The Deck Department is an organizational unit aboard naval and merchant ships. A Deck Officer is an officer serving in the deck department.-Merchant shipping:...

 of a merchant ship. The position is an apprenticeship to become an able seaman

Able Seaman (occupation)

An able seaman is an unlicensed member of the deck department of a merchant ship. An AB may work as a watchstander, a day worker, or a combination of these roles.-Watchstander:...

, and has been for centuries. In modern times, an OS is required to work on a ship for a specific amount of time, gaining what is referred to as "sea time." Once a sufficient amount of sea time is acquired, the OS can apply to take a series of courses, and then a series of examinations to become certified as an able seaman.

An OS is generally not required to stand watch, but must pass examinations on watchstanding skills such as performing lookout duty and being a helmsman

Helmsman

A helmsman is a person who steers a ship, sailboat, submarine, or other type of maritime vessel. On small vessels, particularly privately-owned noncommercial vessels, the functions of skipper and helmsman may be combined in one person. On larger vessels, there is a separate officer of the watch,...

.