Buildtools.jar is not a server file, it is a file that makes a server file.
After downloading the latest buildtoos on the spigot/craftbukkit page of MineOS, you would normally pres the “build spigot” button beside the version you want. Unfortunately there is a minor error there that makes that not work. Workaround here: [resolved] Building Spigot / Craftbukkit fails - solution: update WebUI