DBMS_OUTPUT.ENABLE(NULL);

SET SERVEROUTPUT ON SIZE UNLIMITED;

c:\users\john\home>sqlplus jon/jon

SQL*Plus: Release 11.2.0.3.0 Production on Thu Apr 12 16:23:51 2012

Copyright (c) 1982, 2011, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - Production
With the Partitioning, Oracle Label Security, OLAP, Data Mining,
Oracle Database Vault and Real Application Testing options


Session altered.

orcl> exec dbms_output.put_line('something')

PL/SQL procedure successfully completed.

orcl> exec dbms_output.enable(null)

PL/SQL procedure successfully completed.

orcl> exec dbms_output.put_line('something')

PL/SQL procedure successfully completed.

orcl> set serveroutput on size unlimited
orcl> exec dbms_output.put_line('something')
something
something

PL/SQL procedure successfully completed.

orcl>