Can I define one set of rules good for the Online Viewing Only (OLVO) region and another set of rules for the MAIN region?
Yes. Each of the Dispatch regions (MAIN and OLVO) are going to have their own SYSTEM NUMBER. Typically, a MAIN region is SYSTEM 01 and an Online Viewing Only region is SYSTEM 02. As part of each users external security rules/profile, you can link resource class CACMD and the entity known as DSSIGN to their individual userid's. Typically, a MAIN Dispatch region is accessed through the DSSIGN01 entity while the OLVO region would be accessed through the DSSIGN02 entity (Assuming that these are the system number that you used).
For more information on this topic, please refer to the CA Dispatch 11.7 System Programmers Guide: Chapter 8 - Security and the section entitled Signon Security.