Greek Grammar

...for the vocative case, they form the vocative in -a. Feminine nouns ending in -a and -e follow the pattern of the second declension. Masculine and feminine nouns ending in -os have the same form for the nominative and the vocative, using the long -o, as in the third declension. These are the Attic forms. Those not ending in -os, using the short -o, follow the fourth declension, forming the vocative in -e. Those declined perittosyllabically having an unequal number of syllables, as in the fifth declension, have the same nominative and vocative forms, except for the five established rules.

Nouns declined with a -nt- ending, not having a diphthong in the penultimate syllable, form the vocative by removing the -tos from the genitive. For example, Aias Ajax, genitive Aiantos, vocative O Aian. Charies graceful, genitive charientos, vocative O charien.

Nouns ending in an immutable consonant and barytone not having an acute accent on the last syllable, form the vocative by removing the final -os of the genitive. For example, Hellēn Greek, genitive Hellēnos, vocative O Hellēn. Hektōr Hector, genitive Hektoros, vocative O Hektor.

Nouns ending in -ēs, having the genitive in -eos contracted to -ous, form the vocative in -e and pull the accent back if they are compound and barytone. For example, Demosthenēs, genitive Demosthenous, and [the vocative is] Demosthenes, O Demosthenes.