This was a major problem for me too, i installed the 64bit Oracle driver
"Oracle Database 10g Release 2 (10.2.0.1.0) for Microsoft Windows (x64)" from Oracle download page. This driver i could use from Linked Server in SMO.
Dont forget to check "Allow inprocess" in the provider options, i needed to do this.
Hmm.. i still have som problem when i update data via linked server,
Msg 7333, Level 16, State 2, Line 2
Cannot fetch a row using a bookmark from OLE DB provider "OraOLEDB.Oracle" for linked server "xxxxxx".