A sweet little Unix clone originally developed for the Motorola 6809 CPU. It had loadable/unloadable device drivers. It ran well with 64K RAM and no hard drive. Ported first to the 68000 family, then to many different processors. Used primarily for embedded device control. A version was released for the Radio Shack TRS-80 Color Computer.
There was at one time a plan to standardize school computers across the UK, if not the (then) EEC on OS9. In the UK, this would have meant using the 64K version of the DragonThirtyTwo. But instead, the BbcMicro, and then ResearchMachines PCs became the standard in the UK.
Those of us who used OS-9 on teeny home computers in the early 1980s could never understand why anyone would settle for MS-DOS.
They didn't, they moved on, with systems that moved on, adapted, met business, personal, and technical needs, at levels and volumes far surpassing the imaginations of almost all those who wondered, and did not settle for MS-DOS and/or OS-9.
Most of those who used either were constantly looking for more, and there were those who provided more. More hardware, more software (including improved operating systems taking advantage of the rapid advances made in hardware to provide more computing capabilities), and more efficiency and more productivity as time moved on.
Is there any relation between OS-9 and the ForthLanguage ? No.