PMBOK pg. 38/39. For the exam, there wasn't any hair splitting (as far as I can recall). The way I tried to remember it was that OPAs are things I can virtually checkout from the company library - like my company's policy on social media (it's something my company developed). And that's what the PMBOK says are OPAs "plans, processes, policies, procedures, and knowledge bases specific to and used by the performing organization". So in theory, I could go to SharePoint and download my company's policy.
EFFs are different, they "refer to conditions, not under the control of the project team". For example, stakeholder risk tolerance is an EFF (internal to the project). I can't check it out from the library and the company didn't produce it, it is just there, present, in my project. My company's culture is another great one, it just exists, it wasn't developed (obviously every company tries to groom their culture, but its organic, it's not something we can download).
As for "Personnel administration policies" being an EFF, the PMBOK does not go and define what "Personnel administration policies" actually are, but my guess it is the slew of government regulation(s) regarding working standards and what has to be posted where and when, OSHA type stuff. That being said, the company COULD develop its own HR handbook and this would be an OPA.
Other EFFs that are just there: location of resources (global workforce), government regulations, marketplace conditions, etc.
As best as I can remember from the exam, the EFF v OPA was pretty obvious.