Herbert Weir Smyth, A Greek Grammar for Colleges

VOCATIVE

a. The vocative is never followed immediately by δέ or γάρ.

[*] 1284. In ordinary conversation and public speeches, the polite ὦ is usually added. Without ὦ the vocative may express astonishment, joy, contempt, a threat, or a warning, etc. Thus ἀκούεις Αἰσχίνη; d'ye hear, Aeschines? D. 18.121. But this distinction is not always observed, though in general ὦ has a familiar tone which was unsuited to elevated poetry.

[*] 1285. The vocative is usually found in the interior of a sentence. At the beginning it is emphatic. In prose ἔφη, in poetry ὦ, may stand between the vocative and an attributive or between an attributive and the vocative; in poetry ὦ may be repeated for emphasis.

[*] 1286. In late poetry a predicate adjective may be attracted into the vocative: ὄλβιε κῶρε γένοιο blessed, oh boy, mayest thou be Theocr. 17. 66. Cp. Matutine pater seu Iane libentius audis Hor. S. 2. 6. 20.

[*] 1287. By the omission of σύ or ὑ_μεῖς the nominative with the article may stand in apposition to a vocative: ὦ ἄνδοες οἱ παρόντες you, gentlemen, who are present P. Pr. 337c, ““ὦ Κῦρε καὶ οἱ ἄλλοι Πέρσαι” Cyrus and the rest of you Persians” X. C. 3.3.20; and in apposition to the pronoun in the verb: ὁ παῖς, ἀκολούθει boy, attend me Ar. Ran. 521.

[*] 1288. The nominative may be used in exclamations as a predicate with the subject unexpressed: ““ὦ πικρὸς θεοῖς” oh loathed of heaven” S. Ph. 254, ““φίλος ὦ Μενέλα_ε” ah dear Menelaus” Δ 189; and connected with the vocative by and: ““ὦ πόλις καὶ δῆμε” oh city and people” Ar. Eq. 273. In exclamations about a person: ““ὢ γενναῖος” oh the noble man” P. Phae. 227c.

a. οὗτος is regular in address: οὗτος, τί πάσχεις, ὦ Ξανθία_; ho there, I say, Xanthias, what is the matter with you? Ar. Vesp. 1; ὦ οὗτος, Αἴα_ς ho there, I say, Ajax S. Aj. 89.