PC-MOS/386 is FOSS now, but nobody seems to be interested.
https://github.com/roelandjansen/pcmos386v501
TBH it was a 2nd string product in its prime, IME. DR's Concurrent DOS/386 was the premium offering. I supported it in the late 1980s.
This was around the time that a LAN of cheap PC-XT clones was becoming price-competitive with a big high-power 80386DX machine with lots of RAM and a bunch of dumb terminals on a multiport serial card.
The terminal vendors and multiuser OS vendors fought back with colour terminals and then graphical terminals. CDOS386 supported these. The basic PC design supported 4 RS-232 ports, so you could have an 8MB 386DX with one user on the system console and four users on dumb terminals, all of them able to run even quite demanding DOS apps including fancy stuff like graphical print-preview or a spreadsheet that could draw graphs.
The snag was that CDOS386 was not a DOS; it was a descendant of Concurrent CP/M. It ran DOS apps but could not use DOS drivers – it needed its own. So adding hardware was not a trivial exercise. Kit had to be chosen from short hardware compatibility list (and was therefore expensive) and drivers supplied as object files relinked with the kernel. It was not as simple as modifying CONFIG.SYS and rebooting – PC/MS-DOS sysadmin skills did not transfer over.
And it couldn't usefully run Windows. So as soon as Windows 3 came out in 1990 and the market started to move to Windows and Windows apps, that was largely it for Concurrent DOS, PC-MOS etc.
When DR was bought out by Novell, several forks of the codebase were continued for a while by OEM licencees, and it gained support for FAT32, USB and so on.
A branch of the CDOS codebase is still just about alive though. DR did a 286 version, but were screwed over by Intel. CDOS286 used a feature of prototype 80286 chips to multitask DOS apps, but this was removed in the final shipping CPU hardware. (Intel did not know and put it back but it was too late.)
DR just took the OS in a different direction and sold it as a multitasking real-time OS instead, downplaying DOS compatibility. It bundled a multitasking version of its GEM desktop, too, so it was a multitasking GUI RTOS, called DR-FlexOS.
This found a niche in electronic point-of-sale systems: EPOS, i.e. smart cash registers built around x86 PCs. It was later acquired by IBM who rebranded it as IBM 4680 OS, later IBM 4690 OS. (What a knack for snappy names Big Blue has!)
IBM discontinued it just a few years ago, but it is still sold by Toshiba.