OS/2 was an operating system developed by Microsoft and IBM. It was first released in 1987, and was intended to be a successor to MS-DOS. However, the success of early versions of Windows led Microsoft to abandon development of OS/2. IBM continued supporting the system until 2001. ArcaOS is the current version of OS/2 as of 18 Sep 2022.
NetHack 3.0, released in 1989, was ported to OS/2. Support continued up to NetHack 3.4.3, which remained the newest version of NetHack from 2003 to 2015. The DevTeam doesn't know if NetHack 3.6.6 can run on OS/2.
The Devteam have indicated that they plan to drop support for OS/2. As of 18 Sep 2022, the sys/os2 directory has been moved to the "outdated" section in the development code.
Window ports supported
NetHack on OS/2, like on most other systems, supports the TTY interface. At one time, it was common practice to build X11 support and install drivers to support X11 on OS/2; but the current Xorg distribution does not support installation on OS/2, and the older ones that do are much harder to set up.
The graphical shell on OS/2 is the Presentation Manager. No tile support for Presentation Manager is known to exist.
PDCurses supports OS/2, and it should be possible to build the Curses port on OS/2.
MS-DOS port on OS/2
A TTY-only NetHack for OS/2 is hardly worth the effort: the MS-DOS port runs fine. The native DOS mode does not support the VESA BIOS Extensions; this is not a limitation for NetHack 3.4.3, but is somewhat limiting for NetHack 3.6.6, and will be more so in the future, because the development source has expanded VESA support.
A port of DOSBox is available, and supports VESA BIOS Extensions.