I once talked to a psychiatrist who didn’t know that an octopus has 8 legs. We can’t all know everything.
Octopodes don’t have legs. They have tentacles. 
No, they have arms.
A day or two ago nobody knew the name of Mark Knopfler’s band.
Who?
No, that’s Pete Townsend’s band.
.
If the answer was Pete’s band, would “Who?” be considered a question?
Ok, so how about this one? The Final Jeopardy category was children’s literature. I don’t remember the exact phrasing, but it mentioned the Quadlings, Winkies, and Gillikins, and the contestants were supposed to name the fourth group in a famous fictional country. Now you can all tell me how crazy am I for thinking Munchkins should’ve been easy. I know they were the only ones in the movie and the movie was much more popular than the book, but it’s a classic. None of the read it and made obsessive color-coded maps like I did as I kid? I’ve said too much.
Didn’t get the OPEC one (always sucked at geography) or the Wicked one (never heard of it). Figured out the giraffe one, though – yay, me.
It’s funny you’ve never heard of *Wicked *considering the example you give is about The Wizard of Oz. 
Dire Straits
I only know because they used to be my brother’s favorite band though; I wouldn’t necessarily consider it common knowledge.
You’ve already been on Jeopardy, but if anyone else would like to attempt this little feat, I recommend learning them to the tune of Yankee Doodle Dandy. This is what my 7th grade history teacher made our class do (we all had to individually get up and sing it in front of the class) and now, nineteen years later, I can still do it. The list is so embedded in my brain that I can recite them without resorting to the song, if I don’t want to sound completely ridiculous. Of course, the song in my head only goes up to Bush I, since he was president when I was in junior high, but I can confidently remember who’s been president since without resorting to mnemonic devices.
It’s a good party trick.
With regards to the question in the OP, if someone had said that there was only one African country in OPEC, I would have guessed Libya. With the additional factor that it is also a Commonwealth country, I would have had to think harder, but I think I would have pretty quickly figured out that Nigeria is the correct answer. I know that Nigeria is a Commonwealth country, and I am somewhat familiar with the history of Shell in Nigeria.
Anyone who was educated in a Commonwealth country would likely have gotten the Nigeria question (I actually guessed Nigeria before the extra hint).
What?
No, he’s on second.