The title of “After Twenty Years” is ironic because two old friends are reuniting after a long time, but they are now on opposite sides of the law.

When Bob sees Jimmy, he does not recognize him.  They were young men when they last saw each other.  He explains why he is waiting to Jimmy, thinking he’s just a cop.

It's an appointment made twenty years ago. Sounds a little funny to you, doesn't it?

The ironic element of this is that keeping an appointment from so long ago does make you a good friend.  It is an amazing and sweet gesture, especially for Bob.  Bob had to come all the way from Chicago to keep the appointment, Jimmy stayed in New York.

Jimmy Wells [was] my best chum, and the finest chap in the world. He and I were raised here in New York, just like two brothers, together.

The ironic thing about the title is also that if Bob had not come back, after twenty years of being away, he would never have been arrested.  He was safe in Chicago.  He kept an appointment with the one person who would recognize him and could turn him in.

Yet although twenty years is long enough to change "a good man into a bad one" it is not long enough for either good friend to turn into a bad friend.  Jimmy does turn Bob in, but he doesn’t have the heart to do it himself.  He has another cop arrest Bob.

What is the irony in "After Twenty Years" by O. Henry?

If "Silky" Bob is wanted by the Chicago police he may have decided to move to another big city. The most obvious choice would be New York, because it was the biggest city in America and also because it would seem like a good place for a crook like Bob to operate. We gather from his smooth manner and his nickname of "Silky" that he is some kind of a confidence man. Bob seems supremely self-confident while he is talking to the uniformed cop whom he doesn't recognize as his old friend Jimmy Wells. Bob hasn't been in New York for twenty years. What is especially ironic about O. Henry's story is that in fleeing from the law in Chicago, Bob runs straight into the arms of the law in New York. He is literally walking arm in arm with a policeman when he learns that he is under arrest. He doesn't brag about his misdeeds to the uniformed cop because that would obviously be foolish, but once he thinks he has reunited with his old friend Jimmy he starts confiding about how he has made his fortune.

The two men started up the street, arm in arm. The man from the West, his egotism enlarged by success, was beginning to outline the history of his career. The other, submerged in his overcoat, listened with interest.

The title of the story, "After Twenty Years," can be taken to mean that people can change a lot with the passage of so much time; but it can also be taken to mean that people's sins will catch up with them if they persist in their unlawful behavior long enough. Bob was a big success in the West, and he was proud of his success. He didn't realize, however, that his very success as a criminal could work against him. He had become so notorious that the long arm of the law reached out for him in far-away New York. The fact that he was being arrested in New York on behalf of the Chicago authorities was another sign of his success, along with his diamond scarfpin and diamond-studded pocket-watch.

The irony is that after twenty years one man is now a criminal and the other is a police officer; the exact police officer who is going to take him down and be the cause of his arrest. 

It is also ironic that these men were "best chums" when they were twenty, but now they do not recognize each other. Jimmy, the police officer, says that when Bob lit his cigarette, he recognized him as the person in a wanted poster.  He didn't recognize him in the poster as his best friend but recognized him FROM the poster as a criminal. Bob also does not recognize his "best chum".  He befriends this man who identifies himself as Jimmy.  He even comments that he doesn't remember Jimmy having that height. Still, he is willing to be persuaded that Jimmy grew a few inches after he turned twenty.  It wasnt until he saw his nose, not his full face but his nose, that he realizes that this isn't his friend. 

With irony meaning the contrast between what is expected and what actually happens, this contrast first involves the character 'Silky Bob,' who tells the patrolman who walks his beat that he is just waiting in the doorway of a darkened hardware store for his old friend with whom he is going to have a promised reunion of twenty years.  For, ironically, he is, in fact, speaking to his old friend, who does not reveal his identity because he now is a policeman while his old friend is a wanted man.

It is also situationally ironic that in talking to the officer, 'Silky Bob' lights a cigar, an act which provides enough light for the patrolman to see his "little white scar near his right eyebrow" and recognize him as a man wanted in Chicago.  For, he meant only to arrive at his twenty-year appointment to meet his friend, but, ironically, discovers that time has changed this friend from "a good man into a bad man."

What are examples of irony in O. Henry's "After Twenty Years"?

Irony is usually like a bad joke, a joke that would be funny if it were not sad or painful. The main irony in "After Twenty Years" is in the fact that 'Silky' Bob has traveled a thousand miles and is standing in the cold waiting to meet his old friend Jimmy Wells--who turns out to be a policeman who has him arrested. This is bitter situational irony. No reader would see it as a joke or feel like laughing at Bob. The reader tends to feel sorry both for Bob and for Jimmy. Bob is pained at being sent off to jail and being betrayed by his old friend. He is also pained by the fact that he did so much talking, especially to the arresting plainclothesman, and probably gave away information that could be used against him. Furthermore, he is pained that he considered himself such a slick man but let himself get suckered by two cops. For his part, Jimmy undoubtedly feels regret and some guilt because he felt compelled by duty to have Bob arrested. Jimmy may even feel ashamed of himself for not having had the courage to make the arrest himself. He will remember this evening for many years. The whole story is based on this great irony.

It is ironic that the character traits Bob ascribes to Jimmy Wells turn out to be the traits that will cause Bob to get arrested. Bob tells the unidentified cop:

But I know Jimmy will meet me here if he's alive, for he always was the truest, staunchest old chap in the world....He was a kind of plodder, though, good fellow as he was.

Bob does not realize he is describing the kind of man who might easily decide to become a policeman and who would be so loyal to his oath of duty that he would have to have his old friend arrested. Jimmy would like the routine that goes with being a uniformed cop, doing everything "by the book." He seems happy in his job, and proud of his uniform, judging from the way the narrator describes him in the opening. 

Trying doors as he went, twirling his club with many intricate and artful movements, turning now and then to cast his watchful eye adown the pacific thoroughfare, the officer, with his stalwart form and slight swagger, made a fine picture of a guardian of the peace.

It is ironic that 'Big Joe' Brady's restaurant was torn down. Bob is forced to stay right there because that was the arranged meeting place twenty years before. But he is on the lam and knows he looks suspicious standing in a darkened doorway. That causes him to speak up immediately when confronted by a cop. Jimmy only intended to introduce himself but wasn't given a chance.

“It's all right, officer,” he said, reassuringly. “I'm just waiting for a friend. It's an appointment made twenty years ago. Sounds a little funny to you, doesn't it? Well, I'll explain if you'd like to make certain it's all straight. About that long ago there used to be a restaurant where this store stands— ‘Big Joe’ Brady's restaurant.”

Before Jimmy can tell Bob who he is, Bob has lighted his cigar and revealed that he is the man wanted by the Chicago police. 

All of the ironies in this story would have to be classified as "situational irony," which is always based on an incongruity between characters' expectations and the actual outcome.

Discuss the use of irony in "After Twenty Years" by O. Henry.

O. Henry loved irony, and used it often in his stories, especially in throwing in his famous surprise endings.  Irony sometimes refers to the concept that something that is the opposite of what is expected.  For example, I am an English teacher, and it would be ironic if I was a horrible grammarian.  You wouldn't expect that; because I teach English, you would expect me to know and use grammar correctly.  In "After Twenty Years," there are several unexpected twists to the storyline.  We meet Bob, who by all indications seems to be a very nice guy who wants to meet up with his old friend.  The police officer, polite and fastidious, seems to be only a professional man conversing with a stranger.  All of these assumptions end up being the opposite of what we would have expected.  Bob is really a wanted criminal, on the run from the law.  The police officer really does know the stranger, and isn't merely politely conversing, but confirming his suspicions about the nature of the man waiting on the street.

We learn of all of these ironies in the characters presented at the end of the story, when the police officer turns out to be Bob's good friend, Jimmy.  He knew Bob all along, and was just pretending to be an ignorant inquirer.  We learn that Bob, the seemingly harmless man with a fondness for old friends, is a dangerous criminal.  The largest instance of irony occurs when Jimmy turns in his friend; earlier, Bob had led us to believe that Jimmy was "the truest, staunchest chap in the world."  According to Bob, Jimmy's defining trait as a friend was that he was loyal, and he didn't doubt for one second that he'd show up, keeping his word.  This makes us think that Jimmy is a great guy who would never betray a friend.  Although Jimmy does show up, he unexpectedly betrays his good friend--he put his loyalty to work above his loyalty to his old pal.  The irony exists in the fact that Bob's expectations of Jimmy's kindness and friendship were a bit overrated.

I hope that those thoughts helped; good luck!