This post is a walkthrough of creating a Linked Server to Oracle from 64bit SQL Server.  There's lots of information on doing this on the web, but much of it is out-of-date. 

First, install the correct Oracle drivers.  You want the latest version of the Oracle Data Access Components (ODAC), and you want the XCopy deployment.  They are available here:

64-bit Oracle Data Access Components (ODAC) Downloads

http://www.oracle.com/technetwork/database/windows/downloads/index-090165.html

One you download and unzip this into a folder run the following command from that folder:

C:\Users\dbrowne\Downloads\ODAC121010Xcopy_x64>.\install.bat oledb c:\oracle\odac64 odac64 true

Then you need to add two folders to your system path: c:\oracle\odac64 and c:\oracle\odac64\bin

Then you must reboot for the system path change to be visible by services like SQL Server.

After reboot you're ready to create and test the linked server.

First configure the Oracle OleDB provider to run inside the SQL Server process, and configure it to accept parameters. 

exec master.dbo.sp_MSset_oledb_prop 'ORAOLEDB.Oracle', N'AllowInProcess', 1

exec master.dbo.sp_MSset_oledb_prop 'ORAOLEDB.Oracle', N'DynamicParameters', 1

Then create the linked server definition.  Instead of a TNSNames alias, use an EZConnect identifier.  Here I'm specifying an IP address and a SID to connecto to an Oracle Express instance running on a VM:

 

exec sp_addlinkedserver N'MyOracle', 'Oracle', 'ORAOLEDB.Oracle', N'//172.16.8.119/xe', N'FetchSize=2000', ''

exec master.dbo.sp_serveroption @server=N'MyOracle', @optname=N'rpc out', @optvalue=N'true'

exec sp_addlinkedsrvlogin @rmtsrvname='MyOracle', @useself=N'FALSE', @rmtuser=N'system', @rmtpassword='xxxxxx'     

Now you're ready to test.  We configured the linked server for 'rpc out' so we can send a simple passthrough query to test connectivity:

exec ('select 1 a from dual') at MyOracle

That's it.