Greek Grammar

...and Demosthenes, O Demosthenes.

Nouns ending in -s with a diphthong, or with an -i or -u, form the vocative by removing the -s. For example, pais child, O pai; bous ox, O bou; mantis seer, O manti. However, pous foot forms O pous.

Feminine nouns ending in -o and -ōs, whose genitive ends in -oos, contracted to -ous, form the vocative in -oi. For example, Sapphō, genitive Sapphoos and Sapphous, vocative O Sapphoi.

All others aside from these five—those ending in -ēs through -ē, or -ōs through long -ō, or through -tos, or through a pure -os, and those ending in a double consonant, or an immutable consonant, or two consonants, and simply all that fall outside the five aforementioned rules—have the same nominative and vocative. Nevertheless, aner man, satur satyr, sōter savior, and daer brother-in-law are used.

Concerning the dative plural.

Note concerning the dative plural of the fifth declension: every dative singular ending in a sounded -i, as in the perittosyllabic declension, forms the dative plural by receiving a -s before the -i.