Query execution showing error as login failed
You need to provide password for remote sa account, like this:

EXEC sp_addlinkedsrvlogin @rmtsrvname = 'OPSERVER', @rmtuser = 'SA', @rmtpassword = 'RemoteSaPassword'


Or if you run under local sa account

sp_addlinkedsrvlogin 
  @rmtsrvname = 'OPSERVER', 
  @locallogin = 'sa', 
  @rmtuser = 'sa', 
  @rmtpassword = 'RemoteSaPassword'


FOr windows auth:

sp_addlinkedsrvlogin 
  @rmtsrvname = 'OPSERVER', 
  @locallogin = 'domain\username', 
  @rmtuser = 'sa', 
  @rmtpassword = 'RemoteSaPassword'


Or

sp_addlinkedsrvlogin 
  @rmtsrvname = 'OPSERVER', 
  @useself = 'TRUE'