https://www.soinc.org/ This is the official Science Olympiad website. On this site, you will find all of the rules, updates or changes to events, national tournament information and event descriptions. Under most event descriptions, you can find links to informational resources for that event. Some of these links are very helpful while others are not, but it is worth taking the time to look through them. In addition, there is a store with supplies needed for builds and labs. Be aware that the build materials are not always the best quality, and the top teams often use kits from suppliers other than the official SO ones. In addition, you can often find the lab materials cheaper from other retailers. Plan to start your SO year by printing the official rules and checking out resources available from this site.
https://scioly.org/ The scioly website features a discussion forum and a test exchange. The discussion forum might provide some helpful clarification/direction, but for the most part, it involves just wading through pages of questions and speculations. Far more useful is the test exchange portion of the website. Taking multiple tests prior to competition is one of the keys to being prepared. While the events change from year to year in their focus, there is typically enough overlap that taking tests from previous years is helpful. Be aware that there is no monitoring or oversight of these tests, so you may find errors in them at times. Some of the tests are intentionally written as practice tests and are far harder than the tests that the students will encounter in competition. They are still useful to take to identify weak areas that require further study.
https://scioly.org/wiki/index.php/Main_Page This is a companion page to scioly.org and is not an official SO resource. However, it is a great starting point for an informational overview of the material that you need to know for your event. Like Wikipedia, the information is entered by the general public and not always fact checked. Regardless, it is a very helpful overview – particularly for knowledge events – to get you started. Many students print out the entire wiki page and include it in their binders for review and reference.
https://www.homesciencetools.com/ For any of the supplies you need for lab or build events, homeschool science tools offers an extensive selection of everything science. The costs are very reasonable, and they offer frequent sales if you sign up for the text or email notifications. Consider doing a joint order with other families that need supplies to save on shipping costs.
For event specific resources, please sign in to the members portal.