The vocative is the case of direct address ("O soul"), set off by commas and often preceded by o.
It has the same form as the nominative except in the 2nd-declension singular: -us becomes -e (Brute, domine), -ius becomes -i (fili, Antoni), and meus becomes mi. Plural vocatives are the same as the nominative.
Signals: o before it, commas around it, and a 2nd-person verb or an imperative nearby: te afficis, o anima "you disgrace yourself, O soul".
A noun at the head of a sentence that looks like an address may be in another case: in Ignominia, ignominia … te afficis, ignominia is ablative ("with disgrace"); the person addressed is o anima.