(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.