Nurse O'Brien has "flashing white teeth, a freckled face and an engaging smile". She is well-liked by her patients because of her cheerfulness and vitality. According to Mrs Welman, Nurse O'Brien is good-natured, and does not mind when Mrs Welman snaps at her.
Nurse O'Brien is with Mrs Welman one night, when Mrs Welman asks her to bring her the photograph of Lewis. Nurse O'Brien later tells Nurse Hopkins about this incident.
After Mrs Welman's death, Nurse O'Brien goes to nurse a gentleman who has double pneumonia, at a house called Laborough Court. In the drawing room of the house, she sees the same photograph that Mrs Welman had, of Lewis, and finds out that the person in the photograph is Sir Lewis Rycroft.
When Poirot interviews Nurse O'Brien, she tells him that she had had no suspicions that Mrs Welman's death was not due to natural causes. She says that the thought did come into her head that the tube of morphine which Nurse Hopkins had missed might have been used to poison Mrs Welman, and that if they found morphine in Mrs Welman's body, she would believe Elinor guilty of killing her aunt. She also tells Poirot that she had seen Elinor promising Mrs Welman that she would call the lawyer, and see that everything was done decently, and according to Mrs Welman's wishes. She also says that she saw Elinor looking at Mary one day, and that Elinor had black hate on her face.