(Adapted from a
Quora answer.)
OS/2 1.x was a clean-sweep, largely legacy-free OS with only limited backwards compatibility with DOS.
OS/2 2.x and later used VMs to do the hard stuff of DOS emulation, because they ran on a chip with hardware-assisted DOS VMs: the 80386’s Virtual86 mode.
NeXTstep was a Unix. It predated FreeBSD, but it was based off
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