Questions about Org_id

We are on 11.5.10. We have 3 set of books defined and have 4 org_ids defined 101,102,141,161. If we wanted to define a new set of books where do we actually set up the org_id. I know how to assign operating units and assign org_id etc to a responsibility. I just want to know the navigation steps in 11.5.10 to define a new org_id number eg  org_id 181.