After I had my machine.config and VS 2008 Futures addins completely screwed by the ODP.NET full installation, following turned out to be very non-intrusive and easy to setup option:
Oracle 11g ODAC 126.96.36.199.21 with Xcopy Deployment: http://www.oracle.com/technology/software/htdocs/distlic.html?url=/technology/software/tech/windows/odpnet/utilsoft.html
Only 43MB to download, options for granular install, and only moves files around. "Installation" manual describes few post installation steps required, all about editing environment vars (don't forget to add TNS_ADMIN environment variable pointing into your tns directory)
note: do iisreset after "install" if your data access service is IIS hosted.
One more thing: If you run out of ideas with ORA-12154: TNS:could not resolve the connect identifier specifie, try re-formatting your failing record in tnsnames.ora, if you have all paranthesis etc, ok; Then know, spaces and tabs do matter!!!! And oracle engineers who wrote it, they really suck!! [Update, 16-Feb-2009] Today, the thing that helped was a restart, plsql developer was ok to connect immediately after install, but odp.net only start working after the restart.
Configuration: http://download.oracle.com/docs/cd/B28359_01/win.111/e10927/featConfig.htm What's new from version to version: http://www.filibeto.org/sun/lib/nonsun/oracle/188.8.131.52.0/B28359_01/win.111/b28375/whatsnew.htm Oracle ODP.NET forum: http://forums.oracle.com/forums/forum.jspa?forumID=146 Performance optimisation matters: http://www.oracle.com/technology/oramag/oracle/06-jul/o46odp.html