Here are a few examples:
* "We were to be 'rough and ready' in our methods." (The "r" sound emphasizes the rough and tumble nature of their kidnapping)
* "He was a 'perfect terror,' a 'perfect imp,' a 'perfect devil' and all the perfectest synonyms for the most perfect specimen of the genus 'boy.'" (This uses alliteration to create a sense of exaggeration and comedic absurdity)
* "It was awful, but I couldn't help it, I was forced to laugh." (The "f" sound emphasizes the almost involuntary nature of the laughter)
* "He had the most wonderful brown eyes and the most impish, mirthful grin." (This pairs the "m" sound with the positive connotations of the words to highlight Red Chief's charm)
* "It was a terrible, tortuous trip." (The "t" sound builds the sense of the unpleasant journey)
These are just a few examples, and O. Henry uses alliteration throughout the story to create a rhythm and emphasis. It's a fun exercise to look for these instances as you read!