Hi, I was getting same error when I was trying to create SSRS datasource (SQL RS 2012) to Oracle database. And in my case I found that there were two instances of Oracle client installed on SSRS (SQL Reporting Service) server i.e.
1st Installation (main) at C:\Oracle\*
2nd Installation at C:\Program Files\Oracle*
I resolved this error by uninstalling 2nd Oracle client installation using Oracle Universal Installer utility on SSRS server. I believe when SSRS trying to look for TNSNAMES.ORA file it was not able to refer due to two set of Oracle client.
Thanks,
Ash