1. Edit registry under HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\<Oracle_Home> to make sure NLS_LANG key is set to a valid characterset for the Oracle Client used as the DSN.
2. Please edit the file %OBIEE_Client_Home%\\oraclebi\orahome\bifoundation\server\bin\bi_init.bat to set %ORACLE_HOME%\bin as first entry in the PATH.
3. Please add TNS_ADMIN entry to the bi_init.bat file to point to your Oracle client tnsnames.ora home directory path (in case exists) or to your OBIEE tnsadmin %OBIEE_Client_Home%\Oracle_BI1\network\admin.
e.g.
set TNS_ADMIN=C:\oracle\product\11.2.0\client_1\network\admin
OR
set TNS_ADMIN=C:\Oracle\OracleBIEE11g\Oracle_BI1\network\admin
https://community.oracle.com/thread/2267972