It can be argued that the only humor in "The Open Window" is ironic humor. If one were to analyze Framton Nuttel's escaping flight at the end as humorous, it would be at the expense of (1) ignoring Vera's possibly unkind motives as an enfant terrible (young person intentionally bent on harming weak adults) and (2) ignoring Framton's emotional and mental exhaustion that causes him to need a "rest cure" (does she drive him completely mad and does she hope to?). Since these analytical oversights would be serious ones, critics generally agree that Saki's humor is ironic humor that is meant to satirize groups of individuals (i.e., weak, foolish, and nervous adults) and society as a whole, which is largely made up of foolish, weak, nervous adults.
Ironic humor is found in both Framton's and Vera's remarks. In addition, the narrator expresses some of Framton's ironic thoughts as well. Humor is defined as the quality of being amusing (it's a quality in something). Ironic humor in literature is defined as expressing thoughts contradictory to what seems intended for the purpose of adding humor (i.e., amusement).
An example of this contradictory spirit of humor in irony is expressed in Vera's first remarks to Framton. Her aunt, Mrs. Sappleton, is occupied when Framton comes to call, so Vera says he will have to "put up with" her. "Put up with" is an idiom meaning to make do with something inferior. Vera, this "self-possessed young lady," has not the slightest notion of herself as inferior in any way, yet she says he will have to "put up with her." This is ironic humor: she means the opposite of what she says and intends it to be amusing; she has no expectation that Framton will find her inferior.
"My aunt will be down presently, Mr. Nuttel," said a very self-possessed young lady of fifteen; "in the meantime you must try and put up with me."
There are many such examples, like Framton wondering if Mrs. Sappleton "came into the nice division," meaning he wondered if she would be unpleasant to know, yet the most significant is arguably the last ironically humorous statement Vera makes by way of explanation of Framton's sudden escape. We know that he has been terrified by Vera's ghost/corpse story into flight from the hunters' return, yet Vera ironically supposes he bolted because of the spaniel, which reminds of some event Vera is in the process of inventing. Of course, the only one amused here is Vera and the readers since she and we are the only ones who know she means something quite different from what she says.
"I expect it was the spaniel," said the niece calmly; "he told me he had a horror of dogs. He was once hunted into a cemetery somewhere ... by a pack of pariah dogs ...."
Irony and its humor of contradiction add interest by developing themes, such as the theme of whether Vera is a playful though unthinking innocent or a malicious and spiteful enfant terrible (wicked young person), through the introduction of contradiction and curiosity. It also adds interest by allowing readers to know secrets that characters don't know, such as the secret truth behind Vera's explanation about "pariah dogs." This secret knowledge creates situational irony in which readers know more than all or some characters. It also adds interest by making the content intellectually challenging because readers must sort out the contradictory real truth from the apparent truth and the real motive and intent from the seeming motive and intent.