You'll want to use sp_addlinkedserver to establish a link:
EXEC sp_addlinkedserver 'MakeUpANameForThisLinkToShowInManagementStudio', 'Oracle', 'MSDAORA', 'your oracle server name'
You may also need to create link for the logins:
EXEC sp_addlinkedsrvlogin 'MakeUpANameForThisLinkToShowInManagementStudio', false, 'SQL User', 'Oracle User', 'Oracle Password'
When you query, you do this:
select * from
MakeUpANameForThisLinkToShowInManagementStudio...ServerInstanceName.TableName
http://msdn.microsoft.com/en-us/library/ms190479.aspx