police officer comes by and arrests Soapy, who's sent to prison the next morning. Soapy has been saved from his present predicament, but it's a false salvation.

O.Henry chose this title for his short story because it points to the main sources of the irony that make the story what it is.  The whole point of the story is the irony at the end and it is cops and an anthem that provide the irony.

Throughout the story, Soapy wants to get arrested but can not.  This is a pretty funny situation.  But the situation becomes really ironic right at the end of the story.

At the end, he stands outside the church listening to an anthem he remembers from his youth.  He is inspired by the memory of better times and decides that he will reform his life.  Right then a cop arrests him for vagrancy, just as he had previously wanted.

So, both cops and anthems frustrate Soapy's ambitions in ironic ways.  This is why the story has this title.

It would be helpful if you asked a specific question about this story. Generally speaking, the story is a typical O. Henry one, full of irony. Soapy tries unsuccessfully to get arrested six different ways so that he can spend "three months on the Island," assuring him of a warm place and food during the winter. In each instance, he commits a minor crime, but the results are not what he expects or wants. Just after he hears organ music coming from a church, he decides he will get a job the next day; however, a cop accosts him and arrests him for loitering. A judge sentences him to three months on the island, so ironically Soapy gets his wish after all in this surprise ending, another characteristic of O. Henry's stories.

Another characteristic of the story is O. Henry's use of figurative language. For instance, in the story's exposition, he uses personification and metaphor "Jack Frost's card" to refer to an autumn leaf, a signal to Soapy that he needs to find a place to spend the winter. At another point, O. Henry describes a "street damaged by improvements" in an example of oxymoron.

"The Cop and the Anthem" is both funny and serious. Like O. Henry's story "A Retrieved Reformation," it deals with the subject of reformation and the difficulties people face in trying to reform. There is a passage in Nathaniel Hawthorne's story "Wakefield" that seems to apply to both of the O. Henry stories about reformation. 

Amid the seeming confusion of our mysterious world, individuals are so nicely adjusted to a system, and systems to one another, and to a whole, that, by stepping aside for a moment, a man exposes himself to a fearful risk of losing his place forever. Like Wakefield, he may become, as it were, the Outcast of the Universe.

Once a man has chosen to travel down a certain path for any length of time, it is very difficult for him to turn around and go back. He "exposes himself to a fearful risk of losing his place forever." This is not hard to understand because there is an intense competition among all living creatures for space on this crowded planet. 

A struggle for existence naturally follows from the high rate at which all organic beings tend to increase. There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair.
                                     Charles Darwin, The Origin of Species

Humans cooperate because this is a more effective way of competing for existence, but the competition is always there--competition with the outer world and competition for status and power within the cooperating group.

O. Henry creates an unusual character in Soapy in "The Cop and the Anthem." Soapy is a sort of gentleman-bum. His speech and his manners show that he was once a member of the middle class. For some reason he decided to drop out of the respectable world and live a life of indolence. He may think that he can return to the respectable world any time he chooses, but it seems as if fate has finally turned against him. He can't get arrested when he wants to get arrested, and then he can't turn back when he wants to turn back. Getting arrested and sent to Riker's Island for three months seems like a turning point in Soapy's life. He won't be the same person when he gets out. He will no longer think of himself as a gentleman but as a bum like all the other bums he knows at Riker's Island and in Washington Square. He has had his own private bench in Washington Square for a long time, but he will find it occupied by someone else when he gets out. In this life, as Lewis Carroll shows in Alice's Adventures in Wonderland, we have to keep running just to stay in the same place. 

O. Henry spent three years in prison for embezzlement and never got over it psychologically or emotionally. He wrote under an assumed name and dreaded having his past catch up with him. He was reputed to drink two quarts of whiskey every day, and he died from alcohol-related diseases at the age of forty-seven.