The Basic Operating System (BOS) was an early IBM operating system.

According to folklore, BOS was the predecessor to TOS on the System/360 and it was IPL'd from a card reader. It may have been intended for very small 360s with no disks and limited tape drives.

BOS died out really early as disks such as the IBM 2311 and IBM 2314 became common with the IBM System/360, whereas disks had been a real luxury on the IBM 7090.