The big picture
It's a team thing
Science Olympiad is one of the biggest K-12 STEM competitions in the U.S. Your school puts together a team, everyone picks events, and you spend the season practicing for tournament day.
Some events are written tests. Some are labs. Some are builds where you show up with something you made, like a bridge or a glider. The cool part is you don't have to be good at just one subject. You can be the Codebusters kid, the rocks kid, the build kid, whatever fits you.
It's also not about crowning one "smartest kid." The teams that do well are usually the ones that practice a lot, split events smart, and actually work together.
Divisions
Division B and Division C
Division B is mostly middle school (around grades 6-9, depending on your state). Division C is high school (around grades 9-12).
A lot of the events feel similar across divisions, but the names and difficulty can change. Quick example: middle school has Crime Busters, and high school has Forensics. Same kind of vibe, different event.