MS-DOS 4.0 maybe? It's the only version with multitasking built straight into the kernel.
It has been open sourced a while ago: https://github.com/microsoft/MS-DOS
Windows 3.1?
Since you mention OS/2.
MS-DOS 4.0 maybe? It's the only version with multitasking built straight into the kernel.
It has been open sourced a while ago: https://github.com/microsoft/MS-DOS