I was recently asked by one of my clients about whether we can change the value of the initilization parameter _optimizer_ignore_hints at the session level or not. Well the answer is yes we can change the value of this parameter at session level. Here is a test of it on one of my instance running on 126.96.36.199.0 database version.
SQL*Plus: Release 188.8.131.52.0 Production on Sat Nov 28 22:38:30 2015
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Oracle Database 11g Enterprise Edition Release 184.108.40.206.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> alter session set "_optimizer_ignore_hints" = TRUE ;