In the novel N or M?, Mrs O'Rourke is one of the residents at Sans Souci in Leahampton. She is a large, cheerful woman from Ireland. When Tommy first met her, he observed that she was a "terrifying mountain of a woman with beady eyes and a moustache". On another occasion, Tuppence found her sitting by the window "like some gigantic Buddha". Mrs O'Rourke had a deep, booming voice which could prove quite alarming. For Tuppence, Mrs O'Rourke was "an unholy fascination" with a somewhat sinister smile. She was like "an ogress dimly remembered from early fairy tales". She used to be an antique dealer with a shop named "Kate Kelly's" in Chelsea.
Mrs O'Rourke is highly observant and this proved a threat to the cover that Tommy and Tuppence was trying to maintain. At one point, she noted that Tuppence was coming along well with her knitting yet she remembered that earlier on Tuppence had said that she was no good at it. Another time, Tuppence dropped by mistake the remark "with my own two--" whereupon Mrs O'Rourke quickly cut in and asked if Tuppence had not said that she had three sons earlier on.
In a detailed conversation in the middle part of the novel, Mrs O'Rourke shook Tuppence thoroughly by asking her whether she thought there was "something odd" about the hotel. She said she had noticed that Tuppence had been studying Mrs Perenna, the proprietor. "I've seen you watching her and watching her." Then as for "Mr Meadowes" (Tommy's cover), Mrs O'Rourke said he was puzzling. On the surface a typical Englishman "stupid to the core" but then she would catch a look or a word that shows he is not stupid at all. Then, she added, there are others, including one whose name began with "S". She did not elaborate except to say she did not mean Sophia Minton.
Mrs O'Rourke enjoys sitting by the window in the lounge at Sans Souci, as she is interested in people, and from there she can see who goes in and out of the house, who is on the veranda, and what is happening in the garden.
Mrs O'Rourke tells Tuppence some information about the background of various inhabitants of Sans Souci, such as how Arthur Sprot is a clerk in London who keeps his wife and child at Leahampton for fear of air raids, and how Mrs Perenna is Irish, but claims to be English.
Mrs O'Rourke later invites Tuppence to her room, which is "very untidy" and smells strongly of "peppermint, and stale cake with a faint odour of moth balls added". On every table there are photographs of her children, grandchildren, and other relations.
After Tommy disappears, Mrs Perenna mentions that it is amazing that the police can find no trace of him. Mrs O'Rourke says that the police are only good for fining motorcars and finding people who have not taken out their dog licences. She expresses the opinion that Tommy could have been an enemy agent, because he did not look like a man who had retired from business and had nothing to do, and she thought that he had come to Sans Souci with a purpose.