I might be wrong, but I was under the impression that all of the OS resides on the FD, no matter what? Even little-used portions of it?
A Fusion drive doesn't put OS here, apps there, and data somewhere else again. It looks at which _parts_ of which file are used a lot, and automatically moves things that are used a lot to the SSD part, and things that are used little onto the spinning hard drive.
So all the parts of the OS that you use a lot are on the SSD drive. All the parts that you don't use are on the spinning drive. If you start iTunes, it displays all the album art. So the parts of your music files containing the album art move to the SSD, the parts with actual music stay on the hard drive. You have a huge folder with fonts including huge japanese and chinese fonts - the ones you use are on the SSD drive, the ones you don't use are on the hard drive.