When NNPC was established and under what decree
NNPC's Core Businesses
NNPC Subsidiaries and their operational functions
The Refineries and their various Refining capacities
Which of the NNPC subsidiaries u'll like to work with and why?
Why do you want to work with the NNPC.
Why do you want to work in the Oil industry
Why do you want to leave your former place of work(if you are already working)
Where do you hope to be in 10 years time?
Understand your discipline and how you can apply it in the oil industry
Know the name of the current and immediate past GMD
Expect that you’ll be asked atleast one basic question about your discipline
Know where the head quarters of each of the Subsidiaries are located say Benin, Warri, Lagos etc.
Research the operations of the Subsidiaries so you’ll know which one you can fit in.(I am currently working with the subsidiary I mentioned during my interview so take your time to research,though it does not totally follow).

And of course you may like to prepare as of any other interview.