One Flew Over the Cuckoo's Nest is a social novel or protest novel. Like Upton Sinclair's 1906 novel, The Jungle, or Harriet Beecher Stowe's 1852 novel, Uncle Tom's Cabin, Kesey's novel aims primarily to expose and critique injustice. The main issue it explores is American society's treatment of mental illness. Kesey spent time working at a veterans' hospital and saw firsthand the indignity with which traumatized veterans were treated by the institutions that were supposed to help them. He writes his characters to represent an experience that had long gone ignored by mainstream culture.
The book might also be considered an early example of a postmodern novel. Postmodernism was a social, artistic, and literary movement that arose after World War II and the counter-cultural protests of the Vietnam War. Modernists in the early 20th century had an optimistic belief that they could experiment with form to find new and better ways of expressing the human experience. Modernism was generally aligned with industrialization and the belief that the world was changing for the better.
Postmodernism, on the other hand, was deeply skeptical of industrialization, institutions, and the idea that there could ever be a one-size-fits-all way of representing the human experience. Several bloody wars and a huge amount of trauma in the 20th century convinced postmodernists that the modernists did not have it right. Kesey's novel has some elements inherited from Modernism, such as experimentation with form. For example, he occasionally uses stream-of-consciousness narration to try to represent Bromden's experience. This style came out of modernism. However, Kesey's experimentation with form usually supports the postmodern idea that every individual's experience is subjective and true, even if it is at odds with someone else's experience.
Nowhere is this more evident than in the fact that the entire novel is narrated by a man who is supposedly insane. The novel insists that Bromden's observations and stream of consciousness are his reality, and everyone else has their own reality that intersects with his. Kesey's novel pushes back against institutions that try to homogenize individuals and turn them into mechanical parts of the "Combine." We would all be much better off, the novel implies, if we approached people with compassion and allowed them to be themselves instead of who we expect them to be.