Probably, OS/9. Not to be confused with Mac OS9.
Versions existed for Amiga, Atari, X68000, Macs and lots of other m68k platforms.
But I never came across of the Amiga version.
References:
https://en.wikipedia.org/wiki/OS-9
http://eab.abime.net/showthread.php?t=20376
That would be my vote.
An Australian port, it retailed for $2000 and was aimed primarily at multi-user business applications, since as far as I know it did not come with a GUI.
I have never seen a copy of it, but there is a much easier to obtain version for the Atari ST.
Personally, as the machines my company sold (based on this OS) had a GUI, I'm hard pressed to figure out who was buying the Amiga and ST versions.
Although they would have made a good central server for a point of sales system.
