The tone and the mood of this short story are generally light and humorous.
Throughout this story, the narrator’s words and the dialogue both convey an atmosphere of light amusement. The protagonist, “Young Gillian,” is presented very much as a sort of lightweight twit in the mold of Bertie Wooster from P.G. Wodehouse’s series. Right from the start, he seems like a very frivolous person, making all sorts of jokes about the amount of money that has been left to him. The lightness continues with the narrator’s words. We are told that Old Bryson shows as much interest in Gillian as “a bee shows in a vinegar cruet.” The rest of the story continues in a similar vein, even when the action becomes more serious toward the end. Even as Gillian is being extremely noble, the narration and his words are light. Therefore, the mood of this story is, overall, very light and amused.
As is so often true of O. Henry's stories, the narrative of "One Thousand Dollars" conveys the author's light and ironic tone, as well as his sentimental mood.
(It is important to note that there is a distinction between tone and mood in literature. Whereas the tone of a literary piece has to do with the author's attitude toward the audience, mood touches upon the overall feeling of a literary piece.)
In O. Henry's short story entitled "One Thousand Dollars," the stodgy and disapproving Old Bryson sarcastically suggests how young Gillian can dispense with the one thousand dollars that his rich uncle has left him:
"Why, Bobby Gillian, there's only one logical thing you could do. You can go buy Miss Lotta Lauriere a diamond pendant with the money, and then take yourself to Idaho and inflict your presence upon a ranch. I advise a sheep ranch as I have a particular dislike for sheep."
With verbal irony, Gillian responds to this sarcasm, "Thanks,...I thought I could depend upon you, Old Bryson." Of course, the greater irony exists in Gillian's failed attempt to earn the love of Miss Hayden, an employee of his uncle who has been left only ten dollars. For despite her rejection of his declaration of love, Gillian generously leaves his inheritance of one thousand dollars to her in an envelope containing a fabricated explanation that the old lawyers have found "an amendment" to the will in which the deceased left her this thousand dollars.
The altruistic act of Gillian ironically earns him a greater inheritance because his uncle has been so cynical about his spendthrift nephew that he died with the conviction that Gillian would never make anything of himself. He provided that Miss Hayden would receive his nephew's real inheritance money if Gillian failed to use the one thousand dollars wisely. However, if his disposal of the money were actually unselfish, then Gillian was to be awarded fifty thousand dollars for his improvement in character.
Later in the narrative, "Gillian drift[s] in [the law office handling old Mr. Gillian's affairs] with his air of regarding the world as inconsequent." He hands the lawyers an account of how he has spent the one thousand dollars. However, when Gillian hears that he stands to inherit fifty thousand dollars more if his disposal of the one thousand dollars has been altruistic, he quickly reacts. He snatches the envelope with his accounting of his loving and generous gift to Miss Hayden from the two old lawyers, saying,
"....There isn't a bit of need to bother you with this....I lost the thousand dollars at the races. Good-day to you, gentlemen."
This ironic reversal ends the story with a mood of sentimentality as the young man's altruistic actions have proven his uncle's low opinion of him wrong. It also ensures that his fortune will go to Miss Hayden as an act of love.