How does one do Online database backup

Each tablespace that needs to be backed up must be switched into backup mode before copying the files out to secondary storage (tapes)

Look at this simple example

SQL>ALTER TABLESPACE xyz BEGIN BACKUP;

Then Copy

!cp xyfFile1 /backupDir/

SQL>ALTER TABLESPACE xyz END BACKUP;

It is better to backup table space than to put all table spaces in backup mode.

Backing them up separately incurs less overhead. When done, remember to backup your control files.

Look at this example

SQL>ALTER SYSTEM SWITCH LOGFILE; -- Force log switch to update control file headers

SQL>ALTER DATABASE BACKUP CONTROLFILE TO '/backupDir/control.dbf';

Note: Do not run Online backups during peak processing periods. 

Oracle will write complete database blocks instead of the normal deltas to redo logfiles while in backup mode.

This will lead to excessive database archiving and even database freezes.