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 = 'dbadmin', -- local SQL Server 2005 login
@rmtuser = 'msmith', -- ORACLE Login
@rmtpassword = 'secret' -- ORACLE Password
GO

-- SQL Server linked server setup

exec sp_addlinkedserver 'YOURNEWLINKEDSERVER', '', 'SQLNCLI', NULL, NULL, 'SERVER=15.34.12.5', NULL 

exec sp_addlinkedsrvlogin 'YOURNEWLINKEDSERVER', 'false', NULL, 'username', 'password' 

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Related articles:

How to set up and troubleshoot a linked server to an Oracle database in SQL Server

Microsoft OLE DB Provider for Oracle (Database Engine)