The Dawn of Haiku OS (Spectrum)

It is considerably older on 'normal' hardware. The early BeOS was multi-core in an age when multi-core PCs, and even multi-core workstations, were quite rare.

There architecture on the x86 hardware of the time would have been epic-fail. Even the ringed security architecture of NT 3.5 a few years later was brutal on the hardware at the time [hence the faster but in some ways inferior NT 4.0 that followed].

>and BeOS claims to date back to 1991.

I remember it. And it ran on very proprietary and very expensive hardware. It got some press in the excellent UNIX Workstation magazine and some others [back when there were interesting IT periodicals... sigh].