I have to say I'm not an expert on the DEC OSes, so I won't pretend to know the exact timeline, but the PDP-6 was not a very popular machine, only 23 or so sold. So I would assume it's really PDP-10 TOPS-10 that was the inspiration. OS/8 itself is from 1971 according to wikipedia, but its predecessor is PS/8. And that same article mentions how the interface was patterned after TOPS-10.