So what's left in flash?

Depending on how many directories you move out of flash to the SD card, you could potentially free up a few more megabytes of flash space (check the results by again running df). So what can you do with this space?

The directories that will definitely still be in flash include /boot, /bin, /sbin, /lib and /etc. This means you can still install into flash things like boot-time files, additional configuration files and libraries.

In addition, you're free to create new top-level directories of your own, perhaps to hold the OZ flash images in case you ever need to reflash. In any case, depending on how much you moved onto the SD card, you'll always have more flash space to play with.