I experienced this problem and wanted to share the solution
User getting persistent "ORA-12154: TNS:could not resolve service name" when trying to connect to Oracle when installed in Windows server 2003 user with 64 bit architecture. User tried all recommended steps in the knowledgebase articles like http://www.tableausoftware.com/support/knowledge-base/oracle-connection and is able to successfully TNSPing the database and connect with other Oracle client tools.
If your tableau installation is at C:\Program Files(x86) directory, try moving it/reinstalling it in C:\Program Files.
There is a bug in some versions of Oracle client preventing it from interacting with executables/processes running from the C:\Program Files(x86) directory, which is the typical installation directory for 32 bit apps on a 64 bit machine.
"64-bit Microsoft OS's install 32-bit applications into the following location
"C:\Program Files (x86)\..." rather than the typical location of "C:\Program Files\..."
This causes an existing networking bug to occur where the networking layer is unable to parse program locations that contain parenthesis in the path to the executable which is attempting to connect to Oracle. "
Thus moving your Tableau installation to C:\Program Files allows Oracle/Tableau linking at runtime and should resolve the TNS error.