The speaker addresses someone absent or dead, a thing, a place, or an idea, as if they could hear.
In Julius Caesar
O Julius Caesar, thou art mighty yet! / Thy spirit walks abroad and turns our swords / In our own proper entrails.
Brutus addresses the dead man, "O Julius Caesar," as if he could hear him. The direct call gives Caesar's spirit a presence on the battlefield and turns Brutus's grief into an acknowledgment of the power the dead man still holds.