Although most of the characters in One Flew Over the Cuckoo’s Nest are considered mentally ill, the plot of the novel suggests that there’s a thin line between "normal" and "abnormal." Much of the difference between the normal and the abnormal consists of fear. As one of the patients in the asylum suggests, he could live in the Outside world if only he had the guts. He doesn’t have the guts, so he finds safety in being institutionalized and considered "crazy."
Though Chief Bromden is schizophrenic, it is possible to argue he is one of the sanest people in the book.
Though Nurse Ratched is seen as normal and McMurphy is seen as mentally ill, the nurse is actually the psychopath.