'o appears before a name used as direct address. As we saw it used, it acted a little like a vocative prefix: 'o qeylIS, qeylIS, qeylIS... It seems to be an honorific exclamation, or maybe it's just used for direct address in general and doesn't actually have the implication of deference or reverence that I inferred. If it hadn't been consistently present when the opera "speaks to" Kahless, and consistently absent when it "speaks to" Molor, I might have ignored it as an unimportant background sound. English has almost exactly the same sound with pretty much the same meaning: "O Mouse, do you know the way out of this pool? I am very tired of swimming about here, O Mouse!" (from Alice's Adventures in Wonderland)