In the Royal Navy in the middle of the 18th century, the term Ordinary Seaman was used to refer to a seaman with between one and two years' experience at sea. A seaman with less than a year's experience was referred to as a Landman, and one with more than two years' experience was referred to as an Able Seaman.
Later, the term was formalized as a rating for the lowest normal grade of seaman. They are not trained in any special task. They are required to work at physically hard tasks of great variety. One needs an Ordinary Seaman Certificate to obtain work. One can become an Able Seaman as a promotion from this position.