Execute the following in the query analyzer:
-- add the linked server
EXEC sp_addlinkedserver '<Reference Name>', '<Linked server type>', 'MSDAORA', '<TNS Name>'
-- add the login credentials
EXEC sp_addlinkedsrvlogin '<Reference Name>', '<impersonate - true or false>',<Local User or Null>, '<username>', '<pwd>'
-- Help on the linked server:
EXEC sp_linkedservers
EXEC sp_helpserver
select * from sysservers
If you get error 7399 when you click on tables or query the linked server you need to install the Oracle Net 8 or 9 or whatever client software you need.
If you get error 7314 - remember that everything is case sensitive and all upper case in oracle.