Hello,
That is not something that can be done just using a single script. You will need two business rules and a script. The script will be executed in a business rule triggering Before moving a user and set a certain property value (e.g. CustomAttributeBoolean1 to true). The rule will only be assigned over the users located in the required OU. The second business rule will trigger After moving a user. It will check the property set by the first rule and send an email notification if necessary. Finally, you should have something like the following:
Business rule triggering Before moving a user
$Context.TargetObject.Put("adm-CustomAttributeBoolean1", $True)
$Context.TargetObject.SetInfo()
Business rule triggering After moving a user