Testing an OS, it's quite challenging: defining use-cases is required but since the OS is the base of application workloads, it's not easy to target all the different use-cases. Let's see in this lecture, how we define, write and run tests for Flatcar Container Linux - a community driven OS designed to run container workloads. We will cover the logic of the Kola test framework but also how they integrate in the release cycle of Flatcar. Finally, we will provide some example to run the tests locally and we hope to share some ideas with OS maintainers in order to improve in a global way the way we test Open-Source OS.