You are right. I didn't realize that OSF/1 was used by anyone other than DEC which rebranded their version as Digital Unix and then Tru64 Unix.

The wikipedia page has a link to this Intel Paragon supercomputer which specifically says OSF/1 and Mach 3.0 microkernel.

https://archive.org/details/bitsavers_intelsupergonXPSXPEBro...