Koch was the most consistent jumper over the two rounds in Norway, recording distances of 130.5m and 130m, to accumulate a winning total of 247.3 points.
The 30-year-old’s last win came in Oberstdorf on February 18 while he was also part of the Austrian team that won team gold in Lahti at the start of the month.
German Severin Freund placed second after efforts of 123m and 134m gave him 238.9 and Robert Kranjec completed the podium with a total of 238.
That was just 0.3 ahead of Daiki Ito, the winner from the previous two events, with the fourth-place finish effectively ruling him out of the overall World Cup contention.
He sits fourth with 1066 points but with leader Anders Bardal, who was tenth, on 1295 and only two events of the season left he is now out of the running.
However Andreas Kofler, ninth in Oslo, and Gregor Schlierenzauer, who placed 16th, can still mathematically overtake him but with 1127 and 1172 points respectively it is unlikely.