There are many books available in the market that are designed specifically for Olympiad preparation and can help you prepare for the NSO Olympiad exam. The Skool Plus workbook by ALLEN IntelliBrain is a good set of resources to prepare for the NSO Olympiad exam for kids in grades 1 to 8. It contains crisp theories and a number of practice questions that help to gain a thorough understanding of the concepts and improves knowledge retention.