Not sure about Hawaii, Iceland or New Orleans, but apparently Oslo was Leviathan’s first ever target.
Ouch. I guess Bergen or Trondheim is the capital of Norway now, because I highly doubt the heroes were able to save the city the first time he attacked.
Then again, maybe the remaining people of Oslo were able to rebuild enough to make the place habitable again? This would’ve been, what, 20-30 years ago, probably, so they’d have both time and money to do it by the story’s present day, as long as they could get it properly organized.
It’s not like we haven’t rebuilt cities before, though far smaller ones, probably less damaged than Leviathan would leave Oslo, and during a time where the political environment was far more stable and in favor of the government helping out than it was in the 1990′s and early 2000′s.
One thing that’s interesting regarding Leviathan attacking Oslo is its location. Brockton Bay seems to be straightforwardly located on a simple, accessible coast. Newfoundland and Kyushu were surrounded by the sea. Oslo is also located on the coast, but it’s situated like this:

Oslo City proper is larger than the immediate watery area off its coast. But Leviathan has an insane range – the area in this map is, as far as I can tell, far smaller than Kyushu, and Leviathan was able to hammer at that island from all sides.
So Leviathan would have limited immediately close resources for attacking Oslo, and a bottleneck to go through for an approach/escape by water, but he’d also be in a position to attack a whole bunch of cities from in between them.
Incidentally, out of the five million people in Norway, around two million live immediately around this waterway. By attacking this place, Leviathan would make a very significant dent into the Norwegian population.