Our oral was in June and we had to choose the topic from a pre-selected list which was only available around then, so I couldn't do any sort of preparation speechwise.
However I agree with everyone that essays are excessive as your understanding of the novels/context is simply not enough at this stage and you want to see what teachers like before determining a form/style for your essays (if you're doing conext). I know that you're supposed to write to whatever style you feel the most comfortable in, but a lot of teachers/schools mandate students to write in a certain way and if you deviate from that you will be unfairly marked down.
Also once you start attending class, you'll be exposed to a disccusion-debate style environment where your peers and teacher will have many ideas that you would not have thought of. I found that listening to my peers really helped me formulate new ideas, extend existing ones further so I was ultimately able to decide on which writing style suited me the best. Assuming that you analyse at least two different texts for text response, the same also applies since each teacher/school has a slightly different way of how they prefer students to write a text response.
As for themes I would definitely make as much notes, tabs, highlights of the books as I can because you can bring these into class and share it with your peers to receive feedback and use it to help you write preliminary essays or drafts in class.