In 1939, Oberhauser was assigned to Reichsarbeitsgemeinschaft für Heil- und Pflegeanstalten, part of the office for Action T4 and one of several front organizations of Hitler's Chancellery, in November 1939.

After finishing Action T4 in August 1941, in November Oberhauser was transferred to the staff of the SS and Police Leader (SSPF) for the Lublin district, SS-Brigadeführer Odilo Globocnik, to take part in Operation Reinhard, the extermination of Poland's three million Jews.

[3] For his work in the implementation of Operation Reinhard, Oberhauser was promoted from SS-Hauptscharführer to SS-Untersturmführer, effective 20 April 1943, reaching officer rank in the SS.

Previously, the Reichsführer-SS Heinrich Himmler had made a personal visit to the Operation Reinhard camps, including Bełżec, on 12 February 1943, and had decided to promote Oberhauser.

110 Ks 3/64), his work at Belzec is described as follows (translated to English): The defendant, Oberhauser, – then an SS-Oberscharführer – was on the staff of the senior SS and Police Leader in Lublin from November to Christmas 1941.

When, in spring 1942, a major expansion of Belzec Camp was carried out to increase its capacity for extermination, it was the task of the defendant, to procure the necessary building materials, in particular, for the construction of the larger gas chamber facility.

In exercising his duties, he was aware of the fact that the work carried out with his assistance, was intended to create the conditions for a significant increase in the numbers of those exterminated.

On 1 August 1942, as inspector of the three extermination camps of Belzec, Treblinka and Sobibor, Wirth moved to his new office in Lublin, and succeeded in getting Oberhauser, whom he assessed as a dutiful subordinate, also transferred there.

In addition, he was still available to Wirth, and had to act as an escort on his inspection visits to the extermination camps.After the completion of Operation Reinhard, Oberhauser, along with many of his colleagues, was sent to northern Italy in the group Sonderabteilung Einsatz R to participate in anti-partisan warfare and the deportation and killing of Jews there.

Purportedly, Oberhauser was commandant of Risiera di San Sabba until its closure in late April 1945 (3,000 to 5,000 people died there).

10" of 20 December 1945 (which also served as a basis at the Nuremberg Trials), by his membership in a criminal organization (the SS) and his killing of victims in Grafeneck, Brandenburg and Bernburg.