Using ActiveX Data Objects 2.8 Library as a Reference from Excel VBA, and the connection string is:
\"Driver={Microsoft ODBC for Oracle}.......\"
And I also
I'm pretty sure you need to install Oracle client; what's more, I believe it has to be Oracle 8 Client (MS hasn't updated that driver in years, as far as I know).
But have you tried using Oracle Instant Client? It's pretty painless to install, way, WAY smaller than the standard Oracle client and doesn't interfere with other installations. All you need to do is copy the files to a target directory, set your PATH to include that directory (which can be done inside your app - no need to actually set global PATH if you don't want to), and insert the appropriate registry entry into HKEY_LOCAL_MACHINE\SOFTWARE\ODBC\ODBCINST.INI. We use it for all our ADO clients.
It also has the added bonus of not requiring the maintenance of a TNSNAMES.ORA file; your connect string can simply be user/password@server:port/SID.