PCjs has archived an original copy of what may be the oldest surviving 386-based PC operating system.
Previously, that honor went to (a reconstructed copy of) SCO Xenix 386 v2.2.3c, as explained in this series of OS/2 Museum posts:
Also see this series of SoylentNews posts:
When prompted, use the following serial number and activation key:
Serial number: pcjs.org! Activation key: ylwbrzkd
NOTE: The PCjs SCO Xenix 386 machines are still being debugged, so don’t expect the operating system to work here quite yet.
[PCjs Machine "deskpro386"]
Waiting for machine "deskpro386" to load....