Knut Rød was acquitted from his Holocaust crimes and got to rejoin Norway's police force after the war, because he was useful for spying on communists, plain and simple.
The justification of Rød's acquittal essentially hinged on him being a double agent for the resistance. No evidence was ever found for his claim that he had done anything to warn Jews of impending arrest, and apparently no real specifics about Rød's aid to the resistance was provided either; and the rest of his defense amounted to "he was just doing his job and following orders, he would've put himself and the resistance in danger by refusing to participate in the arrests and deportations"... Some defence, huh!
And somehow, the sixty-year-old anti-communist who told me about Knut Rød got it in his head that Rød "just didn't know" what was going to happen with the deported Jews... Uh-huh, sure, whatever you say.
Anyways, the question in the title.
Knut Rød was the only person charged with collaboration that was acquitted.
Insane as it sounds, Holocaust collaboration was generally not a priority during the settlements, sometimes it wasn't even mentioned as part of the verdict. However, most Holocaust collaborators also worked with the Germans in other ways, which they were much more likely to be punished for. A lot of policemen and stuff definitely slipped through the cracks and got away.