Maybe you can grant the user supervisor rights to the container, then use an IRF to remove the right from the Admin object. I'm not sure; you might not be able to filter the supervisor right.
A more elegant solution is to move Admin user out of that container. This might mean having to use contextful login for the admin user ".admin.org", but that's no big deal.
I find that the best tree design, especially if you are assigning managers of the various containers, is to have the admin be the only user at the top of the tree in the O= container, then create OU subcontainers and put all other users where appropriate.
HTH