The sentence I'm considering is this:
The boy said, "O King, live for ever."
Is the word "O" in the above sentence an adjective?
The sentence I'm considering is this:
The boy said, "O King, live for ever."
Is the word "O" in the above sentence an adjective?
Oh, dear.
It's originally not a word at all, but an "ejaculation" or "interjection"— a (presumedly) spontaneous sound expressive of strong emotion, such as surprise, vexation, joy or grief. It's also used frequently as a "discourse marker" to perform a transition or cover a momentary groping for words.
In the early 12th century, however, English writers adopted the Latin and Old French use of O to signal that the following noun phrase is a "vocative"—that is, that the noun designates the person addressed. In the course of the evolution from Early Modern to Modern English it became conventional to spell the sound ‹Oh› when used as an interjection or discourse marker, reserving the bare ‹O› spelling for vocative uses, or to lend an archaic or exotic or "poetic" air.
It is not at all clear that vocative ‹O› has ever been anything but a literary form in English, except among people who conscientiously imitate literary use—there seems to have been a lot of this in the 19th century, and you occasionally hear it employed ironically by English professors. I'm quite certain I've never heard it used spontaneously.
So it doesn't really fit anywhere in the conventional parts of speech. If I had to assign it to a category I'd call it a literary proclitic; but I'd rather not.
There happens to be a specific technical term for that word. You're looking at the vocative article.
Authorities seem to disagree as to whether articles are adjectives. I happen to agree with those that say they are.
Almost in all grammar books, the last or eighth part of speech is "interjection". O is an interjection used in the entence presented to express a sudden strong emotion (invocation or wish).
Yes, it is an interjection. Just like the "O" in "O, what fun it is to ride!" or "Oh, I see."