What rights do you have on the domain?
You may need to take over the old admins domain account to make sure that you can see the hidden OU.
You should be able to see all the OUs as a domain admin, but if the admin is sneaky enough he could hide an OU from you.
unfortunate that you have someone in your organization that is upset enough to do that. this is another case that if you are at the point that you need to let your IT staff go you must sever ties immediately and not let them linger around with admin rights. it will be tough to get around for a while but these problems are what happen when you let them stay. the above info is what you need to try. reset his account password and login as him. that way you would have his rights to the domain. he more than likely added the “domain admins” group to the ACL list of the OU and account and then put a DENY permission on the OU and account. so no matter what rights the group has they still can’t see it.
what you also need to do is get on the main router and change the password. and also check that router to see what accounts have VPN access to the network from outside. then reset ALL of those account passwords. more than likely he knows them or has them written down since he is being let go. better safe than sorry. in that case if something does go wrong you know he didn’t get in that way.
also you need to set all accounts on the domain to expire the passwords so everyone has to change their passwords. this is not going to go over well but you need to explain to management that he has already put in place items to circumvent your security and you need to fix them.