The speaker addresses someone absent or dead, a thing, a place or an idea, as if they could hear.
In Romeo and Juliet
O brawling love, O loving hate!
With the repeated "O," Romeo addresses love and hatred as if they could hear him. The exclamation gives his outburst the tone of declamation, of someone speaking to concepts rather than to a person.
Act I: The brawl and the ball, Romeo
O serpent heart, hid with a flowering face!
Act III: Mercutio, Tybalt, and the banishment, Juliet
Romeo, Romeo, Romeo! Here's drink. I drink to thee.
Act IV: The potion of apparent death, Juliet
Is it e'en so? Then I defy you, stars!
Act V: The tomb and the reconciliation, Romeo
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