1. Under RRS 89.1 you can have more than one party make up the OA as long as they all comply under 89.1 (f). You can fully define in a definitions section of the NOR.
2. Depending on what type of event, I'd do this as a separate memorandum of understanding between the multiple parties as by the time you get to the NoR publication - all parties representing the OA are covered by the mere fact that it's the OA that publishes the NoR and is an entity upon itself.

Just my 2c - Nicole B.

I would add that I think from the competitors' perspective the OA should be a monolithic entity, even if it's composed of more than one organization. Any division of responsibilities and functions should be worked out internally within the OA member organizations. 

How formal that division of responsibilities needs to be is probably dependent on the individual members and the nature of the event - some events a handshake agreement may be sufficient, some events may require legally binding paperwork.

When I was Class President of the Viper 640 Class Association, we gave a lot of thought about the definition of the OA for our World Championships etc. On the one hand we wanted the Class Association to be part of the OA because we wanted to ensure a consistent standard of event and we brought a great deal of money and sponsorship to the event. On the other hand we wanted the host club to feel empowered to do their best work.  There is a lot of value to the OA be a single legal entity and the host club often has the right levels of insurance coverage to host an event and protect its volunteers. The Class also has its own D & O insurance. However when you create a new joint entity between club and class, then you risk weakening the insurance coverage of both organizations.   Thus what we do is we agree that the host club is the OA with the proviso that (1) The OA's organizing committee include at least one class representative (2) The budget is reviewed by the Class before and after the event (we contribute financially and have a responsibility to our members) (3) The NoR and SIs are approved by he Class.   We are extremely careful not to interfere with the initiatives and independence of the volunteers and we allow the host club to run the event. We choose our hosts carefully and then allow them to do their best work and bring local flavor and the pride of the host club to the event.  We have a book of past budgets, past SIs and past NoRs, access to judges and PROs who are familiar with the class ....so we see our role as available to help, to answer questions as needed, to provide contacts as needed for judges and sponsors.. Occasionally we might tactfully point something out that the host club might not have thought about and we provide an event guide for PROs (with a preface that emphasizes that it is only a guide)  We have had some amazing events produced by some amazing hosts over the years and, honestly, it has been a privilege to work with every host club during my time as Class leader . 
I note that some classes still require that they are the OA for their major championships or set up a joint body to run an event.  It would not be the route that I would choose, but if it works, it all good.      

Note how the Helly Hansen Sailing World Regatta Series (formerly the NOOD series) handles it at the top of their NOR…

https://yachtscoring.com/event_documents/14802/2022_Annapolis_3_Day_NOR_Final.pdf