Working example of OPENDATASOURCE command using SQL Authentication
To run it with a SQL login, just need to specify User ID and Password in the connection string (called "init string" in BOL)

SELECT
    * FROM
OPENDATASOURCE (
   'SQLNCLI', -- or SQLNCLI
   'Data Source=OtherServer\InstanceName;Catalog=RemoteDB;User ID=SQLLogin;Password=Secret;').RemoteDB.dbo.SomeTable