You can set the isolation level for a transaction (with SET TRANSACTION) and for a session (with ALTER SESSION).
If INIT.ORA parameter SERIALIZABLE is set to TRUE the isolation level is set to SERIALIZABLE for the instance. Oracle recommends not to change the default value (FALSE) for the SERIALIZABLE parameter. It is provided for sites that must run in ANSI/ISO compatible mode, or that want to use applications written to run with earlier versions of Oracle.