O. Henry

He liked twist endings in his stories and his mustache

William Sydney Porter, Pen Name O. Henry, is an American writer of chiefly short fiction (the most famous piece being "The Gift of the Magi") and one novel (Cabbages and Kings). His stories are famous for their Mandatory Twist Endings, warm characterization and wit.

For some inexplicable reason, he is most popular in the former USSR republics, where phrases such as "Bolivar cannot carry double" have become stock quotes.

The "Oh Henry!" candy bar (later associated with Hank Aaron) was partly named in homage to him. (And partly named for a boy who flirted with the girls at the candy factory.)

Works by O. Henry with their own trope pages include:

"The Gift of the Magi"

Other works by O. Henry provide examples of:

All Just a Dream: The Roads We Take, although it is more a case of Ironic Echo.
Banana Republic: Cabbages and Kings is the Trope Namer.
Beleaguered Childhood Friend: After 20 Years.
Big Applesauce: A popular setting of many of his stories; the short story collection The Four Million is set there.
Bratty Half Pint: Johnny Dorset from The Ransom of Red Chief.
Can't Get in Trouble For Nuthin: The Cop and the Anthem.
Con Man: Jeff Peters and Andy Tucker, protagonists in a cycle of stories.
Cowboy: Normally of the Working Cowboy varieties, protagonists in many stories.
Corrupt Corporate Executive: An early example in "Shark" Dodson from The Roads We Take.
Domestic Abuse: A Harlem Tragedy, which, despite the title and the subject matter, is a very light-hearted story.
Downer Ending: Occasionally, for example in The Furnished Room.
Exasperated Perp: The Ransom of Red Chief.
Ill Girl: The Last Leaf.
Ineffectual Sympathetic Villain: The Ransom of Red Chief.
Luke I Am Your Father:
Mock Millionaire: "Transients In Arcadia"; "The Policeman O'Roon"
No Name Given: What the hell does that "O" stand for?
Pity the Kidnapper: The Ransom of Red Chief.
Stupid Crooks: As mentioned immediately above, the kidnappers in "The Ransom of Red Chief." They're dumb enough to kidnap an obviously evil child, and he's such a terror that they end up having to pay his father to take him back.
That Man Is Dead: A Retrieved Reformation, in a way.
Train Job: The Roads We Take.
Trick Twist
Villain Protagonist: Because O. Henry spent time in jail, many of his stories, like The Ransom of Red Chief, focus on (relatively low-time) criminals.
Wild West: Another popular setting. Is usually limited to Texas ranches.