Johnny Prefers to Be With Captors (Situational Irony)
Contrary to the reader's expectation that a ten-year-old boy would be frightened when two strangers abduct him from his front yard, Johnny prefers to stay with Bill and Sam in their cave hideout. In this instance of situational irony, O. Henry inverts the standard narrative about kidnapping by having the captive wish to stay with his captors rather than return home to the dull reality of living with his father and going to school.
Bill Doesn't Know Johnny Is Behind Him (Dramatic Irony)
When Sam returns to the cave after posting the ransom letter, he discovers that Bill and the boy have vanished. Bill emerges from the bushes and informs Sam that he freed Johnny, releasing him on the road to Summit. As Bill explains his reasoning, Sam sees that Johnny is in fact standing eight feet behind Bill, grinning. In this instance of dramatic irony, Sam, Johnny, and the reader know a crucial piece of information—that Johnny has not returned to Summit—while Bill remains comedically oblivious.
The Kidnappers Pay Dorset to Take Back His Son (Situational Irony)
In response to Sam and Bill's ransom demand of $1,500, Johnny's father tells the kidnappers that they "are a little high in [their] demands." He makes a counter-proposition to "bring Johnny home and pay me two hundred and fifty dollars in cash, and I agree to take him off your hands." In this instance of situational irony, the kidnappers' expectation that a father would be concerned for the welfare of his son and pay to have him back is undermined by the revelation that Dorset knows how exhausting it can be to look after Johnny. The irony is extended when the kidnappers agree to the man's terms; they even consider Dorset generous for only asking $250.