O'Connor once said she could only write one type of sentence (source: The Habit of Being, p. 69). There's a bit of truth to that. Many of her sentences are bare bones consisting of: subject, verb, some modifiers, and nothing more. We don't find many descriptive words or complicated structures in her writing. In addition, there's not much that feels clipped (like Hemingway) and certainly nothing flashy or unconventional in the postmodern vein. Nope, just plain, meat and potatoes sentences. You can really notice this in paragraphs without dialogue, where one short, simple sentence after another begins with the subject:
He and the grandmother discussed better times. The old lady said that in her opinion Europe was entirely to blame for the way things were now. She said the way Europe acted you would think we were made of money and Red Sam said it was no use talking about it, she was exactly right. The children ran outside into the white sunlight and looked at the monkey in the lacy chinaberry tree. He was busy catching fleas on himself and biting each one carefully between his teeth as if it were a delicacy. (44)
See what we mean?
The simple construction of the narrator's sentences complements the colorful speech of O'Connor's characters. Notice that O'Connor often fully writes out the accents and regional speech of her characters (read anything The Misfit says for an example of this). It also enables O'Connor to use the written equivalent of deadpan delivery when she wants to be funny.