Onomatopoeia in "O madness of discourse"

While writing a commentary for Shakespeare's Troilus and Cressida, my classmate and I got into a small quarrel over the classification of an onomatopoeia.

We were wondering, for the line “O madness of discourse”, would O (which I guess is equivalent to the more modern Oh) be classified as an onomatopoeia? My classmate claims that it’s nothing more than repetition, but I still believe it’s the former.

Onomatopoeia means that a word for a sound imitates that sound. For example, jingle is the word for a metallic sound, and it imitates that metallic sound.

O is not onomatopoeic because it is not the word for the sound oh.

O or Oh is simply used as an exclamation. It is not an onomatopoeia since this refers to the property of a word sounding like the sound it is referring to, such as the use of quack or moo.

The word O, sometimes spelled Oh, can as here in the Shakespeare quote above serve as a vocative particle. The OED gives its first sense as:

1. Standing before a sb. in the vocative relation.

The etymology is interesting:

A natural (or what now seems a natural) exclamation, expressive of feeling. OE. had neither ó!, nor á! (which would have phonetically given ME. ô!). Not in OHG., or early ONor.; in Goth., prob. from Greek; in MHG. and later (Christian) Norse, prob. from Latin. In early ME. 12th c., app. from L. (or ? Fr.); but often varying with A!, esp. in northern writers. Wyclif has O (or A) only when O is in the Vulgate. In OE., Lat. O was rendered by lá or éalá.

Colloquial English doesn’t use a vocative oh much. It sounds old-fashioned.

O Death, where is thy sting? O Grave, where is thy victory?

But you still find it in poetry, of course:

1850 Tennyson In Mem. iv. 5 ― O heart, how fares it with thee now?

When spelled O, it must be capitalized; when spelled oh, it is only capitalized when it falls at the start of a sentence.

There is another, interjective sense of oh that we do still use colloquially.

Oh, I should think so!

The OED says of this second sense:

2. In other connexions, or without construction, expressing, according to intonation, various emotions, as appeal, entreaty, surprise, pain, lament, etc. In this use, in 17th and 18th c., often written oh (q.v.); but this form is now usual only when the exclamation is quite detached from what follows (see oh); ^{_{O being used with an imperative, optative, or exclamatory sentence or phrase, as in O take me back again! O would I were there! O that I might see him! O for another glimpse of it! O the pity of it! O dear me! O dear! O me!; often also in O yes, O no, O indeed, O really, and the like.}}

And here are some more Tennyson citations: