To make trace files visible to all users, set the parameter _TRACE_FILES_PUBLIC=TRUE, this is undocumented parameter. This parameter requires a restart of instance to take into effect.
SQL> alter system set “_TRACE_FILES_PUBLIC”=true scope=spfile;
System altered.
Shows the trace file is accessible by all users, you may have change permissions on the user_dump_dest folder
-rw-r–r– 1 oracle oinstall 1433 Feb 21 21:07 test_ora_26729.trc
View the following link to find if parameter can be modified without restart or requires a restart.
http://atomic-temporary-2592919.wpcomstaging.com/2008/05/14/internal-parameters/