There is a need to search on all changes made to objects in an OU recursively.
Create a new search and follow these steps:
1. Click the drop-down next to the "Add" button on the "WHAT" tab.
2. Select "Subsystem" | "Active Directory", this will load the AD object picker.
3. In the AD Object picker, select the "This object and child objects" (one level below the OU), or "This object and all child objects" (all objects under the OU recursively) radio button.
4. Navigate to the OU you want to search on, select it and click "Add".