I appreciate the correction. Do you mean it was the first in the sense that "TOPS-10" was the new name for the PDP-6 monitor program as it was developed moving forward, and that all the PDP-7 and PDP-8 operating systems came long after the -6, or more literally that TOPS-10 itself is where the inspiration for OS/8 came from? Or do you not consider OS/8 an OS?
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.
