Here is a list of the vocative forms for all noun
declensions:
Declension
Singular
Plural
1st
puella
puellae
2nd -us
colone
coloni
2nd -ius
fili
filii
3rd
canis
canes
4th
senatus
senatus
5th
dies
dies
Very often, an "O" will appear immediately
before the vocative.
In all cases but two, the
vocative is identical to the same number of the nominative. In
the case of the 2nd declension, the singular vocative will be -e
instead of -us for nouns ending in -us and -r (e.g. "O colone"
for the noun colonus, -i, m. and "O vir" for vir, viri,
m.) and long -i- for nouns ending in -ius (e.g. "O Fili"
for filius, -i, m.).
