Oslo was destroyed several times, and after the fourteenth calamity in 1624, the king ordered it rebuilt at a new site across the bay . By "destroyed", of course, we mean the normal sort of "destroyed" where people can escape and the land is left intact if a bit scorched or soggy or something. Of course, that's only the non-mad Oslo's history, which is all we can reference accurately, and the Oslo of Agatha's history may have seen radically different Sparky activity.
Due to its advantageous position, Oslo was a major sailing and shipping center; its shipyards may well have been Stockholm's greatest competitors.
At some point within fairly recent memory, the entire city and a great bit of the surrounding geography was erased from the face of the earth, leaving a very symmetrical crater.
One of the few survivors of the tragedy was Othar Tryggvassen — but if he caused it, he hasn't made the fact widely known. However, there are suspicions the disaster is linked to the Great Wall of Norway, which was definitely built by Tryggvassen .
The Great Wall of Norway lies along the Swedish border east of the Crater of Oslo; it starts at the Skagerrak and extends for quite a remarkable distance, 500km or more, heading towards Trondheim. What the Swedes think of it is unknown, but they seem to have left it largely alone.