How does one install Oracle Spatial?

Oracle Spatial can be installed via the Oracle installer, DBCA or from the command line interface. Look at these examples:

For Oracle 9i and 10g:

SQL> connect SYS as SYSDBA
SQL> @?/md/admin/mdinst.sql

For Oracle 8i:

SQL> REM # Create MDSYS users as part of the ORD (Object Relational Data) 
SQL> REM #    install script ordisys.sql:
SQL> connect SYS as SYSDBA
SQL> @?/ord/admin/ordisys.sql

SQL> REM # Install MD catalog into the MDSYS user
SQL> connect mdsys/mdsys
SQL> @?/md/admin/catmd.sql

For Oracle 8.0:

SQL> REM # Create user MDSYS with admin option
SQL> connect SYS as SYSDBA
SQL> @?/md/install/mdsysid.sql

SQL> REM # Install MD catalog into the MDSYS user
SQL> connect mdsys/mdsys
SQL> @?/md/admin/catmd.sql

NOTE: Also run catmd.sql after upgrading to a new Oracle release.