We first encounter the "Combine" in chapter 1 of One Flew Over the Cuckoo's Nest when "Chief" Bromden argues that shaving before breakfast is the "worst time" to shave. He explains:
When you got something under your belt you're stronger and more wide awake, and the bastards who work for the Combine aren't so apt to slip one of their machines in on you in place of an electric shaver.
He then goes on to describe how he has no chance against the "machines" when forced to shave before breakfast, because all he will be faced with is screaming faces "trapped" behind mirrors.
This mysterious reference to the "Combine" is later explained as "a huge organization that aims to adjust the Outside as well as [the "Big Nurse"] has the Inside." From this perspective, we can understand that Bromden sees the "Combine" as a complex tool of oppression in which patients at the mental health ward are manipulated, controlled, and managed. This authoritarian giant forces the mental patients into submission, but according to Bromden, it is also present int he outside world, such as the artificial damming of the waters in his ancestral hunting grounds.