Description:

This package contains $data which lists possible values of $^O along
with description for each. It also provides some helper functions.

Tags
* unix

Unix-like operating systems. This currently excludes beos/haiku.

* bsd

BSD-derived Unix operating systems.

* sysv

SysV-derived Unix operating systems.

* posix

For POSIX-compliant OSes, including fully-, mostly-, and
largely-compliant ones (source:
<https://en.wikipedia.org/wiki/POSIX>).

From what I can gather, dec_osf is not POSIX compliant, although
there is a posix package for it.