Working in Active Directory Users and Computers (ADUC) trying to delete an Organizational Unit (OU), you get the following error :
Active Directory Domain Services
You do not have sufficient privileges to delete < Organizational Unit >, or this
object is protected from accidental deletetion.
By default, you need to uncheck the box “Protect object from accidental deletion”. To achieve this, you need to activate Advanced Features on ADUC console.
Click on : View > Advanced Features
Now you got a lot more options!
Right click on the OU you want to remove and then Properties ;
Click on Object tab ;
Uncheck the box Protect object from accidental deletion.