Kindly mention which class NSO and IMO you are talking about.
Practising NCERT and your school books question is advisable. Download previous year questions, and some preparatory books ussually MCQs of science. When talking about IMO, since most of the portion is mathematics, practising many books as well as other competitions papers can be an option. There are several other exams such as NTSE, NSTSE, RMO(for 10th), and many more. It is important to remember that the topics will be the same as that of the school syllabus however the level will be upgraded, therefore, try practising tricky and more logical questions of that topic.
For NSO, science is involved learning NCERT will be extremely beneficial especially for biology and chemistry related questions.
Try solving Mental ability questions from legit books, and forget to solve sample papers and previous year papers. Download from here:
hope this helps