There are a number of significant events and actions that could signify this novel's climax. Normally the climax of a novel represents an action or event of high drama that results in changes to the eventual outcome for all important characters. Such a climax is usually comprised of a character's decisive action, although sometimes a random occurrence that affects the plot can be considered a climax.
One event that changes everything is the suicide of Billy, brought about when Nurse Ratched tells him she will tell his mother about his sexual activity with a prostitute (encouraged by McMurphy). Billy's behavior,m his shyness and stammering, are completely altered when McMurphy takes him under his wing. McMurphy thinks Billy doesn't belong in the asylum and so tries to help him. McMurphy's anger is so great at Nurse Ratched when Billy's dies that he physically attacks her, trying to kill her. He is punished with a lobotomy, thereby assuring he will have to stay in the asylum permanently. This sequence of events represents a dramatic climax resulting from previous character decisions and actions.
McMurphy tears open Nurse Ratched's uniform exposing her breasts. This is climactic because Ratched hates McMurphy for being unable to control him. When McMurphy exposes her breasts, the other patients observe the transgression in awe. Since McMurphy has crossed a line of control and submission, Nurse Ratched orders him to be lobotomized, making him much easier to control.
There is another slight rise in action when Chief Bromdan kills McMurphy and escapes from the hospital.