a manual backup of a datafile using o/s. How RMAN will know about it?

Ans. You have to catalog those backups with RMAN using catalog.

Whenever we take any backup through RMAN, in the repository information of the backup is recorded. The RMAN respository can be either controlfile or recovery catalog. However if I take a backup through OS command then RMAN does not aware of that and hence recorded are not reflected in the repository. This is also true whenever we create a new controlfile or a backup taken by RMAN is transferred to another place using OS command then controlfile/recovery catalog does not know about the prior backups of the database. So in order to restore database with a new created controlfile we need to inform RMAN about the backups taken before so that it can pick one to restore. 

This task can be done by catalog command in RMAN. With catalog command it can
-Add information of backup pieces and image copies in the repository that are on disk.
-Record a datafile copy as a level 0 incremental backup in the RMAN repository.
-Record of a datafile copy that was taken by OS.

But CATALOG command has some restrictions. It can't do the following.
-Can't catalog a file that belong to different database.
-Can't catalog a backup piece that exists on an sbt device.

Examples of Catalog command
1)Catalog an archive log: To catalog two archived logs named /oracle/oradata/mac/arc001_223.arc and /oracle/oradata/arju/arc001_224.arc the command is,
RMAN>CATALOG ARCHIVELOG '/oracle/oradata/mac/arc001_223.arc', '/oracle/oradata/arju/arc001_224.arc';

2)Catalog a file copy as an incremental backup: To catalog datafile copy '/oradata/backup/users01.dbf' as an incremental level 0 backup your command will be,
RMAN>CATALOG DATAFILECOPY '/oradata/backup/users01.dbf' LEVEL 0;
Note that this datafile copy was taken backup either using the RMAN BACKUP AS COPY command, or by using operating system utilities in conjunction with ALTER TABLESPACE BEGIN/END BACKUP.

3)Catalog multiple copies in a directory: To catalog all valid backups from directory /tmp/backups issue,
RMAN>CATALOG START WITH '/tmp/backups' NOPROMPT;

4)Catalog files in the flash recovery area: To catalog all files in the currently enabled flash recovery area without prompting the user for each one issue,
RMAN>CATALOG RECOVERY AREA NOPROMPT;

5)Catalog backup pieces: To catalog backup piece /oradata2/o4jccf4 issue,
RMAN>CATALOG BACKUPPIECE '/oradata2/o4jccf4'; 

Uncatalog Backup
In many cases you need to uncatalog command. Suppose you do not want a specific backup or copy to be eligible to be restored but also do not want to delete it. 
To uncatalog all archived logs issue,
RMAN>CHANGE ARCHIVELOG ALL UNCATALOG;

To uncataog tablespace USERS issue,
RMAN>CHANGE BACKUP OF TABLESPACE USERS UNCATALOG;

To uncatalog a backuppiece name /oradata2/oft7qq issue,
RMAN>CHANGE BACKUPPIECE '/oradata2/oft7qq' UNCATALOG;