An ordinary seaman (OS) is a member of the deck department of a 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". For centuries, the term ordinary seaman was used to refer to a seaman with between one and two years' experience at sea, who showed enough seamanship to be so rated by their captain. Historically, in some navies and the merchant marine, a sailor with less experience was called a landsman.
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. Thus an OS will often be found on a ship's bridge after working hours taking a turn at the ship's wheel or being familiarized with bridge equipment.
