Why are I and O always capitalized, but a is not?

There are three single-letter words. They are the article a, the pronoun I, and the interjection O. The pronoun I and the interjection O are always capitalized, but the article a follows normal capitalization rules. Why is this the case? When and where did this originate?

Edit: As described in the answers to Question 7988, the pronoun I was first used in the 13th century, to avoid confusion with the dotless j, and was retained as a typographic convention because i "gets lost" due to its small size. However, this does not explain why O is always capitalized, but a is not, since a and o are approximately the same size.

Solution 1:

English capitalization is a can of worms nestled inside the larger swamp of English spelling -- i.e, there is almost never any good reason for why anything is spelled, punctuated, or capitalized any particular way -- they just are. Sometimes.

However, one contributing reason for this particular feature is probably the fact that both I and O are usually stressed (as one can tell from the fact that they contain diphthongs, which require stress to avoid reduction), while a is never stressed and therefore always reduced to /ə/.

Solution 2:

The difference between a and O is that O, as an interjection, represents a lightly more independent linguistic unit. You might say:

O! That’s a great plan.

but not

Hey, that's a...! Sorry, I just like indefinite articles.

Hence a has a more subtle effect that doesn’t modify the meaning substantially, but rather needs other parts of speech to complement it.

Solution 3:

Oh! is an interjection. (I disapprove of using O for Oh!) For me, O is not an interjection, but could be called a case marker for the vocative case; that is, a noun used in direct address. In "O John, come here," O goes with John and could be called a preposition, as prepositions substitute for terminations on nouns which are commonly considered as case markers. For example "John's" uses 's, a case ending for possessive case. But "of John" is equivalent, using a prepostion for the same function. O indicates direct address, but also is emphatic. Perhaps it was thought that using a capital for O indicated emphatic or "pay attention" for which function capital letters are sometimes used, with pedants claiming "That is shouting."