An ordinary seaman (OS) is an unlicensed member of the deck department of a merchant ship. The position is an apprenticeship to become an able seaman, 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.