Deirdre
Coroner
I'd like to give the writers the benefit of the doubt on this one, and assume that Greg's names for his grandparents aren't quite so formal. I know a lot of people have strange names for their grandparents, since they come up with names at such a young age. I'd guess that Greg just started calling his grandparents Nana and Papa Olaf at a really young age, before he could really make the connection that Hojem was their last name and Olaf was the first name. Or, perhaps, still thinking along those lines, his grandparents' last name really is Olafsen or something like that, but Olaf was easier for a toddler Greg to say, and is, as a result, what he stuck with. I mainly say this because I know my family abounds with strange names for grandparents. You can check http://www.thenewparentsguide.com/grandparents-nicknames.htm to see just how much variety there is in grandparent names.
Good explonation but I've never had doubts about Nana - surely it was granny, not her name. Nana even here is used. The only problem is Olaf which is in 100% male name. Maybe writers just made it 'cause it sounds Scandinavian but don't really care that Olaf is a male name?