I have just shift applications to 64-Bit Windows server 2012 on a machine which were running before on Windows server 2008. In Win 2012 Microsoft OLEDB for Oracle is not available. Oracle client 11gR2 is installed.
I'm not sure whether Win2012 comes with MSDAORA or not as I don't have one handy to check, but the "usual suspect" here is that MSDAORA only comes in 32 bit (they didn't port it to 64 bit). As such, you can either use it with a 32 bit app (even though the OS is 64 bit), or use an OLEDB provider that DOES come in 64 bit (such as Oracle's - OraOledb.Oracle).
Note that there's a known issue that affects an 11203 install where the Oracle provider doesnt get registered by default (although it's installed). Just run regsvr32 on oraoledb11.dll to fix that.