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.
With dbca you can easily drop an oracle database.
However you may wish manually to delete a database. The following steps will help you to manually drop a database.

1)Set the Oracle SID of the database which you interested to drop.
On my system it is,

$export ORACLE_SID=dbase

2)Connect to SQL*Plus as SYSDBA privilege.

$sqlplus / as sysdba
SQL*Plus: Release 10.2.0.1.0 - Production on Tue May 13 02:41:24 2008
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options

3)Mount the database.

SQL> startup force mount

ORACLE instance started.
Total System Global Area 167772160 bytes
Fixed Size 2019288 bytes
Variable Size 92274728 bytes
Database Buffers 67108864 bytes
Redo Buffers 6369280 bytes
Database mounted.

4)Enable restricted session.

SQL> alter system enable RESTRICTED session;
System altered.

5)Drop the database.

SQL> drop database;
Database dropped.