i also preparing for PMP so let me put down my understanding
EEF : are mostly constraints for your project, thing that you may affect you project though you might be able to influence these but nor completely eliminate them. Again these are things that prove as constraints
eg: ORg Processes, or more confusingly the PMIS, why is PMIS a EEF because it is something you have to use on your proj. A company standard. No 2 ways abt it/
OPA: Are data/info or Assets owned by your organization which will help you while the Project Lifecycle. Eg : Historical Data, Templates.
OPA are non compulsory, but under best practices you are suppose to refer them to help you in the project Life cycle