Thank You for your response.
I forgot to mention in my earlier post that i don't have a separate oracle client installed on my machine.I just have Tableau Desktop with Tableau Oracle Driver (from Tableau support site). I did little research on internet they all are suggesting to modify sqlnet.ora and tnsnames.ora which come with Oracle client installation i believe.
I was able to connect to Teradata (using Tableau Teradata driver though) but not Oracle.
Appreciate any help you can provide on this...