I had to look it up, found this:
In the late 1980s, Sun Microsystems, the major BSD vendor at the time, entered into a partnership with System V developer AT&T. Together, they integrated major BSD and SunOS features into System V to create System V, revision 4 (SVR4). In response, a number of other Unix vendors, including IBM, Hewlett-Packard, and DEC, formed the Open Software Foundation (OSF). OSF was dedicated to the creation of open architecture systems (specifications that were not under the control of any single company).
OSF/1 was one of the fruits of this effort. Based on Carnegie Mellon’s BSD-derived Mach and integrating features of both major Unix flavors, it was released as an alternative to SVR4. It conforms to Unix standards and is also compliant with the System V Interface Definitions (SVID). OSF/1 is not a complete operating system but specifies many operating system components that may be used when developing an operating system. As such, OSF/1 is as much a specification as it is an implementation. Operating systems that it has influenced include HP-UX, AIX, and especially Tru64 UNIX (formerly known as Digital UNIX and DEC OSF/1).