To take online backups the database should be running in Archivelog mode. To check whether the database is running in Archivelog mode or Noarchivelog mode. Start sqlplus and then connect as SYSDBA.
After connecting give the command "archive log list" this will show you the status of archiving.
$ sqlplus Enter User:/ as sysdba SQL> ARCHIVE LOG LIST
If the database is running in archive log mode then you can take online backups.
Let us suppose we want to take online backup of "USERS" tablespace. You can query the V$DATAFILE view to find out the name of datafiles associated with this tablespace. Lets suppose the file is
Give the following series of commands to take online backup of USERS tablespace.
$ sqlplus Enter User:/ as sysdba SQL> alter tablespace users begin backup; SQL> host cp /u01/ica/usr1.dbf /u02/backup SQL> alter tablespace users end backup; SQL> exit;