Just to clear things up a little...
There's really only been one name change in OSF's long history - when Compaq bought Digital. Digital UNIX was originally based on OSF sources, so calling Digital Unix 3.x "OSF/3" is akin to calling a PlayStation a "PSX". They're not different versions. They're two names for the same thing.
When Compaq bought DEC, they renamed Digital UNIX to Tru64. Digital UNIX 4.x (OSF/4) became Tru64 UNIX 4.x (still OSF/4). Tru64 UNIX 5.0 is OSF/5. That's all. Simple, right?
I've also never heard anyone call Tru64 "Compaq UNIX", and I do work for Compaq, in the Alpha division. I guess you could call it that, because it is Compaq's flavor of UNIX. But again, it's just another name for the same thing, not a name change.
So maybe the question should be, "What kind of OS has more names than Satan"?