In "The Song of Roland," after many (but not all) of the laisses, there comes a puzzling marking: AOI. I've seen it explained in several ways, out there on the Internets:
- An exclamation like the joyful medieval "Eya!", although (given the number of different emotional contexts it occurs in) I would think it would have to be something like our modern "Oh!" which takes on many meanings depending on inflection.
- A mistaken writing of "Am", short for "Amen."
- A notional relating to music.
- A notation relating to performance (e.g., perform only the AOI laisses for a slightly shorter performance).
- An exhortation to the audience to "Listen!"
- A copyist's mark noting which laisses came from a version considered canonical by the copyist, and which came from other versions.
I have one more possibility to add.
If you sing "AOI" as "aou-EEE," on a rising fifth, it sounds very much like a horn. You know, like the most famous medieval fictional horn, which features exactly in this chanson.
I think this ranks as more plausible than someone mis-spelling "Amen" dozens of times, but not as plausible as a copyist's or performer's notation.