As mentioned above , hot backup is taken tablespace by tablespace:

steps:

1. take a list of tablespaces and corresponding datafiles you want to take backup.

2. Issue the command ALTER TABLESPACE tablespace name BEGIN BACKUP;

3. Copy all the datafiles corresponding to this tablespace to the backup drive.

4. Issue the command ALTER TABLESPACE tablespace name END BACKUP;

Repeat the above for all the tablespaces. It's advisable to take backup at the time of low db activity..

Pands