If by "rebuild OS," you mean you need to rebuild the RAID1 system array, then you don't need to reinstall the OS at all. Simply replace the failed drive and initiate the rebuild. The OS won't have changed at all. The rebuild simply mirrors the first drive to the replacement drive.
If by "rebuild OS," you mean you need to reinstall the OS but that the RAID1 system array is fine and does not require rebuilding, then yes, the data on the RAID5 array will be accessible after the OS is reinstalled.
In the second case, you will, however, need to take ownership of the folders/directories on the data array on behalf of a user or users that exist on the newly installed OS because the newly installed OS will not recognize the user account references from the prior OS even if they have the same names (Windows uses lengthy user ID numbers behind the scenes).