The old PC-MOS was released under the GNU GPL this Summer. From Wikipedia:

PC-MOS/386 was a multi-user, computer multitasking operating system produced by The Software Link (TSL), announced at COMDEX in November 1986 for February 1987 release.[1] PC-MOS/386, a successor to PC-MOS, can run many MS-DOS software titles on the host machine or a terminal connected to it. Unlike MS-DOS, PC-MOS/386 is optimized for the Intel 80386 processor; however early versions will run on any x86 computer.

The GitHub project includes a 1.44MB disk image for the latest version that will work under VirtualBox, but does not include older versions of the operating system from before it required an 80386+. The system won’t work properly if you set a modern date at the boot up prompt.