"O" is an adjective and vocative in English. What does it mean?

Answer 1

"O" as an adjective I am unfamiliar with.
Used as an interjection to indicate the vocative case, as in "O ye of little faith!"
"O" is the third person singular pronoun for all he, she, it...but thats in other languages.

Answer 2

I haven't heard of "O" as an adjective in English. It is an honourary prefix in Japanese, though.

"Vocative" refers to calling someone or addressing them. For example, when I am addressing a Supreme Court Justice (in Canada), I will say "If your Lordship turns to page 36 of the reasons in ..," but when I am speaking about a Supreme Court Justice, I will say "As his Lordship (her Ladyship) said earlier today, my client need not answer that question." Some languages, such as Czech, will change their nouns and pronouns depending upon how they are used, so these words will change when used in a vocative sense.

I am not sure what "O" has to do with vocative other than being used in the sense of "O Scotty, beam me up."