Wikipedia
Oberarzt (short: OArzt or OA), literally meaning "senior physician," in English known as first lieutenant (Dr.), was a military commissioned officer rank in the Austro-Hungarian Common Army until 1918, and in German Reichswehr and Wehrmacht until 1945.
It describes a qualified or licensed surgeon or dentist with a rank equal to 1st lieutenant (de: Oberleutnant) or sub-lieutenant lieutenant (de: Oberleutant zur See) NATO-Rangcode OF1a in anglophone armed forces.
While holders of this rank were commissioned officers, their authority was usually limited to medical matters and medical staff, and they were regarded as non-combatants and accorded the status of " protected persons" under international humanitarian law when participating in humanitarian work during armed conflicts. As such, they could not be attacked, harmed or taken as prisoners of war, and were entitled under the Geneva Conventions to carry out their work without being inhibited.