Oscarsborg is the name of the fortress protecting the seaward approaches to Oslo, Norway. Her guns have only been involved in one battle, and the two biggest guns only fired once each in anger.
The ship was the German cruiser Blücher. The first hit took out the Blücher's forward fire control station, effectively disabling the ship's forward guns. The second hit took out the aircraft hangar, igniting aviation fuel and infantry munitions stored on deck.
These were the only two rounds Aron and Moses were to ever fire in anger, their inexperienced crews and long reload times effectively taking them out of the battle. If you only get one shot - make it count.