After you setup a linked server to a remote ORACLE database, you have to set the remote login and password for the connection to work:

EXEC sp_addlinkedsrvlogin
@rmtsrvname = '34.55.230.99', -- ORACLE IP or Network Name
@useself = false,
@locallogin = 'sa', -- local SQL Server 2005 login
@rmtuser = 'msmith', -- ORACLE Login
@rmtpassword = 'secret' -- ORACLE Password
GO