How does one do Offline database backup

Shutdown the database from SQLPLUS or Server Manager. 

Backup all files to secondary storage (e.g. tapes). Ensure that you backup all data files, all control files and all logfiles.

When completed, restart your database.

Do the following queries to get a list of all files that needs to be backed up

SQL>Select name from sys.v_$datafile;

SQL>Select member from sys.v_$logfile;

SQL>Select name from sys.v_$controlfile;

Sometimes Oracle takes forever to shutdown with the "immediate" option.

As workaround to this problem, shutdown using these commands

SQL>alter system checkpoint;

SQL>shutdown abort

SQL>startup restrict

SQL>shutdown immediate

Note that if your database is in ARCHIVELOG mode, one can still use archived log files to roll forward from an off line backup. 

If you cannot take your database down for a cold (Offline) backup at a convenient time, switch your database into ARCHIVELOG mode and perform hot (Online) backups.