It is not required to back up the operating system on storage servers. Oracle does not support installing any additional software on the storage servers, so installing backup agents is out. Each storage server has an internal USB stick with a recovery image that matches the software version of the cell. It also contains the relevant configuration files for the cell, so in the event that the OS on a cell becomes corrupted, the cell can be reimaged in little time. Only perform a reimage of a cell at the behest of Oracle support.
When performing a cell reimage, you are given the choice of removing the existing data from the cell or keeping the data. If you have data on existing disks, then choose to keep the data, and import the celldisks after the restore is complete.
For more information on the cell recovery, see chapter 7 of the latest Exadata Owner's Guide.