This completely compelling story by O'Connor is filled with possible themes to explore in a literary analysis essay. It is important to remember that a good essay is making an argumentative claim (thesis) and then writing an essay in which you prove that to be true with evidence from the text. In order to focus your essay you need to think about which aspects of the story are most compelling to you and then go from there. Are you drawn to a particular character? Then ask yourself what that character's thoughts and actions contribute to the overall meaning of the novel. Are you interested in a specific topic? Then you need to ask yourself which events or characters help contribute to your understanding of that topic in the novel.
For example: If you are interested in Mrs. Freeman as a character then ask yourself what does she add to the story? Why does the story start and end with her? How does she serve as a contrast to the other characters? Your ANSWER to the question will be your thesis statement. You could do this to create a thesis statement about any of the characters, but it makes sense to write about one of the four major characters.
If you are interested in themes and meaning then ask yourself "what truth of life did I learn by my reading this story?" You certainly learn about the hypocrisy of human nature, and the difference between intelligence and common sense (book smarts vs. street smarts). You learn about how people define themselves and how their world view shapes their interactions with others. Choose one topic and then discuss how the characters and events of the story work to make that point.
If you need more topics, I would encourage you to look at the themes page here on enotes.