Here are the sample questions that were used in previous years' competitions:
https://science.osti.gov/wdts/nsb/Regional-Competitions/Resources/MS-Sample-Questions
https://science.osti.gov/wdts/nsb/Regional-Competitions/Resources/HS-Sample-Questions
And here is the table of textbook resources as provided by the Department of Energy:
Preparation Tips for Science Olympiad:
https://www.soinc.org/preparation-tips
Resource Guide for NOSB: