This is a faitly simple query, where you would first query the Department Org-Unit deeply in order to get all the Org-Units in the structure, and then you would simply find all the Organizational Processes linked to the Org-Unit.
The querybelow will find all org-units in the department structure and find the Org. Processes linked directly to any of the Org-Units.
Select [Org-Unit] Into @org Where [Name]=&"Root" Or [Aggregation of] Deeply &"Root" Select [Organizational Process] Where [Org-Unit] in @org
If the org-Units are part of participants the query will be slightly more complicated.