There could be multiple reasons for such issues. There might be multiple installation of Oracle in the machine and that could sometimes cause issues.
Did you checked your TNS entry file? are correct entries available there? Try creating an Oracle ODBC connection through windows data sources and check if it is working fine. If so, then use the same ODBC in Qlikview as source.
Also, try creating oracle OLEDB connection from Qlikview and check if it is working.
Thank you for your prompt response as I was away during the time. Also for calling the office with regards to this question.
Well, I had looked thru and yes, I think it's because of the multiple connection. I have thus changed the TNS_ADMIN to point to the desired tnsnames.ora and it's now working.