In O. Henry's "The Gift of the Magi," the theme of the story is that of selfless giving from the heart, like that of the magi or wise men in the Christmas story. The irony, of course, is that Della sells her hair to buy Jim a watch fob ("fob chain") for his pocket watch, but Jim sells his watch to buy Della beautiful combs for her long, luxurious hair. In this case, each has sacrificed what was most dear to him or her for the other—which the other then cannot use.
irony