In poetry, an apostrophe is a figure of speech in which the poet addresses an absent person, an abstract idea, or a thing. Apostrophes are found throughout poetry, but they’re less common since the early 20th century. Poets may apostrophize a beloved, the Muse, God, love, time, or any other entity that can’t respond in reality.
The word O is often used to signal such an invocation.
Some examples:
When will you ever, Peace, wild wooddove, shy wings shut,
Your round me roaming end, and under be my boughs? [Gerard Manley Hopkins]O holy virgin! clad in purest white,
Unlock heav’n's golden gates, and issue forth; [William Blake]O cunning Love! with tears thou keep’st me blind,
Lest eyes well-seeing thy foul faults should find. [William Shakespeare]
Walt Whitman wrote about the death of Lincoln.
O CAPTAIN! my Captain! our fearful trip is done; The ship has weather’d every rack, the prize we sought is won; The port is near, the bells I hear, the people all exulting, While follow eyes the steady keel, the vessel grim and daring: But O heart! heart! heart! 5 O the bleeding drops of red, Where on the deck my Captain lies, Fallen cold and dead. 2
O Captain! my Captain! rise up and hear the bells;Rise up—for you the flag is flung—for you the bugle trills; 10 For you bouquets and ribbon’d wreaths—for you the shores a-crowding; For you they call, the swaying mass, their eager faces turning; Here Captain! dear father! This arm beneath your head; It is some dream that on the deck, 15 You’ve fallen cold and dead. 3
My Captain does not answer, his lips are pale and still;My father does not feel my arm, he has no pulse nor will; The ship is anchor’d safe and sound, its voyage closed and done; From fearful trip, the victor ship, comes in with object won; 20 Exult, O shores, and ring, O bells! But I, with mournful tread, Walk the deck my Captain lies, Fallen cold and dead.