Unlike backups of writeable tablespaces, you do not need to mark the beginning and end of the online backup of a read-only tablespace. Using the ALTER TABLESPACE BEGIN and END BACKUP commands with a read-only tablespace causes an error.
After you change a read-only tablespace to a read-write tablespace, you need to resume your normal backups of the tablespace, just as you do when you bring an offline read-write tablespace back online.
Bringing the datafiles of a read-only tablespace online does not make these files writeable, nor does it cause the file header to be updated. Thus it is not necessary to perform a backup of these files, as is necessary when you bring a writeable datafile back online.