Source : Free On-Line Dictionary of Computing

Basic Operating System
     
         (BOS) An early [when?] {IBM} {operating
        system}.
     
        According to folklore, BOS was the predecessor to {TOS} on the
        {IBM 360} and it was {IPL}'d from a {card reader}.  It may
        have been intended for very small 360's with no disks and
        limited tape drives.
     
        BOS died out really early [when?] as disks such as the 2311
        and 2314 became common with the {IBM 360}, whereas disks had
        been a real luxury on the {IBM 7090}.
     
        (1999-01-29)