TOPIC: "Personnel Administration", EEF or OPA?

PMBOK(6th edition) mentioned it as part of OPA on page 102(4.4.1.5), but mentioned it as EEF on page 135(5.1.1.3).

It's actually both. Personnel Administration is a broad category, elements of which can act as inputs to various processes.

For instance, knowing who in particular is certified or experienced in a technology needed for the project is an OPA element, whereas legal and/or company policy directives around how employees must be treated would be an EEF eleement.

I hope that helps clarify!

Tracy