I have had a look at this and think I have found something which meets your needs.
First of all, I setup a triggered action which updates the Org Unit if the Owner field is changed.
Search and Link: $([Employee#.EmployeeAssociatedCI]OrgUnitLink_RecID)
I then setup a read-only rule on the Org Unit form element: $(!IsNull(Owner))
I have excluded Department from the above examples, but the process would be the same.
In terms of forcing the Department choice, that would be done by validating the field using a pick list. This can be a static list or you can constrain based on values in the current record.
I would be happy to provide further details as to any constraints if desired- please let me know what values you would like to appear in the pick list (e.g. based on OU or perhaps another field)
Hope this helps, Donna
sorry I found this confusing and it did not work for me. I appreciate your time however.
I'm not sure whether I understand your question correct but if you main problem is to make the "Organizational Unit" field read-only if the "Owner" field is set, I would expect that a Read-only rule for field "OrgUnitLink" with the expression
$(nvl(Owner, "") != "")
would do the job.