One of the ways is this:

[oracle@srv3 ~]$ ps -eaf | grep pmon
 oracle   10435 10414  0 10:22 pts/1    00:00:00 grep --color=auto pmon
 oracle   31892     1  0 06:49 ?        00:00:00 ora_pmon_ORCL
 [oracle@srv3 ~]$ sqlplus / as sysdba
 SQL*Plus: Release 12.2.0.1.0 Production on Tue Jan 22 10:22:55 2019
 Copyright (c) 1982, 2016, Oracle.  All rights reserved.
 Connected to:
 Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production
 SQL> drop database;
 drop database
 *
 ERROR at line 1:
 ORA-12719: operation requires database is in RESTRICTED mode
 SQL> startup force mount exclusive restrict
 ORACLE instance started.
 Total System Global Area 1207959552 bytes
 Fixed Size                  8620080 bytes
 Variable Size             436209616 bytes
 Database Buffers          754974720 bytes
 Redo Buffers                8155136 bytes
 Database mounted.
 SQL> drop database;
 Database dropped.
 Disconnected from Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production
 SQL> exit
 [oracle@srv3 ~]$ ps -eaf | grep pmon
 oracle   10623 10414  0 10:24 pts/1    00:00:00 grep --color=auto pmon
 [oracle@srv3 ~]$