The "Links" section of the Cantus Planus website sent me to the Gregorian Chant site at Princeton (which I already knew about, but which I haven't visited in a long while).
That site sent me to the website Cantus, at the University of Western Ontario. And there I went to the Search page, and entered "O mundi domina" in the search box.
I got back at least 40 results, several of which sent me to pages with photgraphs like this one:
And that, my friends, is obviously a very old chant book. And there, right in front of our eyes, are the little-known medieval "O" antiphons - "O Gabriel," "O Rex Pacifice," "O Ierusalem," along with the first few opening notes of "O Mundi Domina" - I mentioned just one post ago.
Here's a different "O mundi domina," in the older musical notation:
In a different vein, here's another link: Office chant books from the Cathedral Church Augsburg, found on a German site. Here is a page of Responsory verses from that Cathedral.
Wonderful stuff out there these days - and how great that it is available to all!