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.