"The Open Window" is told from a third-person narrative perspective, and the narrative voice has a gentle, humorous, witty tone to it. For example, when the main character, Framton, tells his hosts about his illness, Saki writes:
Framton . . . laboured under the tolerably widespread delusion that total strangers and chance acquaintances are hungry for the least detail of one's ailments and infirmities, their cause and cure.
The tone of phrases like "laboured under the tolerably widespread delusion" seems to be sarcastically condescending but in a gentle, playful way. The tone is very reminiscent of P. G. Wodehouse, a well-loved English humorist, who wrote sentences like, "He had the look of one who had drunk the cup of life and found a dead beetle in the bottom," and "He had just enough intelligence to open his mouth when he wanted to eat, but certainly no more."
In Saki's short story, the humorous tone is also evident at the end of the story. After we discover that the tale the niece told Framton, which made him flee in terror from the house, was entirely made up, Saki writes, "Romance was her speciality." The tone here is one of dry, comic understatement.
There is also in the story a rather suspenseful, macabre tone while we, along with Framton, believe the niece's tale about the three men who died in the marsh, supposedly three years ago to the day of this story. Introducing the tale, the niece declares that, "Her [Mrs. Sappleton's] great tragedy happened just three years ago." She then points to an open window and says to Framton, "You may wonder why we keep that window wide open on an October afternoon," before explaining that the poor Mrs. Sappleton labors under the impression that one day the three dead men will come home, through that open window. Framton later notices that Mrs. Sappleton's "eyes were constantly straying past him to the open window and the lawn beyond," and finally, towards the end of the story, the three men and their dog emerge from the twilight, carrying their guns and walking slowly towards the open window. This "ghastly" tale creates a suspenseful tone, which, however, is soon undercut by the revelation that it was all a fanciful, somewhat cruel trick designed merely to frighten the restless Framton.