OS/2
https://encyclopedia2.thefreedictionary.com/OS%2f2+NT
Also, Wikipedia "OS/2 NT" redirects to Windows NT.
The good news is that if you are feeling particularly nostalgic and have a 16 or 32 bit processor (or emulator), you can relive the OS/2 glory days with ArcaOS from https://www.arcanoae.com/arcaos/ It'll cost you about $130 for a personal license, but if you want it, it's still there, and still supported.
(Still waiting for the x86-64 VMS port to be completed... https://www.vmssoftware.com/products_roadmap.html)
As we all know, transposing each initial by 1 from VMS gets you WNT. Rather like the HAL/IBM Space Odyssey factoid.