SQL> alter session set isolation_level = READ COMMITTED;
Session altered.
SQL> select sid, name, isdefault, value from V$SES_OPTIMIZER_ENV where name like '%isolation%' and sid = (select sys_context('USERENV', 'SID') from dual);
SID NAME ISD VALUE
289 transaction_isolation_level YES read_commited
SQL> alter session set isolation_level = SERIALIZABLE;
Session altered.
SQL> select sid, name, isdefault, value from V$SES_OPTIMIZER_ENV where name like '%isolation%' and sid = (select sys_context('USERENV', 'SID') from dual);
SID NAME ISD VALUE
289 transaction_isolation_level NO serializable