​​Oppenhoff was born on 18 August 1902, and from 1933 was a lawyer in Aachen, known for defending priests and Jews. The Nazis were unhappy with Oppenhoff and he had been warned by high-ranking National Socialist officials several times because of his actions. Oppenhoff also worked for the armaments company Veltrup, which helped him avoid being drafted into the military. In September 1944, when the American troops were near Aachen and the city was largely destroyed, Oppenhoff reached out to the American forces. After the withdrawal of German troops on 31 October 1944, he was appointed as mayor by the military.

Oppenhoff knew that, as long as the war was not yet over, his life was in danger. Despite this he chose to cooperate with the victorious powers. Unfortunately, on 25 March 1945 he was murdered in front of his home on Eupener Straße by a Werewolf-Kommando squad. The five attackers included a 14-year-old boy and a 16-year-old girl. Although the attackers were caught, the exact individual responsible for the kill was never discovered, but Oppenhoff was surely a victim of the National Socialist dictatorship. He was buried in the Aachen Eastern Cemetery.

To remember him, a monument made of 13 concrete pillars was built in his memory, with the following quotes:

​"Judge not, that ye be not judged." - New Testament 

​"Man is free. The individuality of man is transformed into freedom." - G. Büchner 

​"Love thy neighbour as thyself." - Leviticus 

​"Silence can become a crime." - Kurt Schumacher 

​"Our generation is a guilty generation to a very great extent." - Alfred Oelp 

​"The dignity of man is inviolable." – §1 Grundgesetz (Basic Law Germany) 

​"One can only overcome fate through understanding." - Hermann Hesse 

​"Not only the deed but also suffering is a path to freedom." - D. Bonhoeffer 

​"The imprisoned truth torments." - Rosa Luxemburg 

​"Sixth Commandment: Thou shall not kill." - Hebrew Bible 

​"If he has not fought alone and is defeated, the enemy has not yet won." - Berthold Brecht / C. v. Ossietzky 

​"Love your enemies, bless those who curse you." - New Testament​