How to DROP an Oracle Database

You can do it at the OS level by deleting all the files of the database. The files to be deleted can be found using:

1) select * from dba_data_files;
2) select * from v$logfile;
3) select * from v$controlfile;
4) archive log list
5) initSID.ora
6) In addition you can clean the UDUMP, BDUMP, scripts etc

Clean up the listener.ora and the tnsnames.ora. make sure that the oratab entry is also removed

However this is not a good way to drop an Oracle database. you should use dbca utility to drop the database. or for Oracle 10g and higher you can use drop database command

SHUTDOWN IMMEDIATE;

STARTUP RESTRICT MOUNT;
DROP DATABASE;