Ordinary Seaman is the lowest normal grade of sailor. 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-Bodied Seaman as a promotion from this position. A sailor is a member of the crew of a ship or boat. ... Ordinary Seaman Certificate is a required certification to obtain a job as an Ordinary Seaman. ...