The vocative is used in exclamations and in direct address:
ὦ Ζεῦ καὶ θεοί
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,
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.
In late poetry a predicate adjective may be attracted into the vocative: ὄλβιε κῶρε γένοιο
By the omission of σύ or ὑ_μεῖς the nominative with the article may stand in apposition to a vocative: ὦ ἄνδοες οἱ παρόντες
ὦ Κῦρε καὶ οἱ ἄλλοι Πέρσαι
The nominative may be used in exclamations as a predicate with the subject unexpressed:
ὦ πικρὸς θεοῖς
φίλος ὦ Μενέλα_ε
ὦ πόλις καὶ δῆμε
a. οὗτος is regular in address: οὗτος, τί πάσχεις, ὦ Ξανθία_; ho there, I say, Xanthias,
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].