[Laurent Rizzotti]

My own table compiled from various sources:

Obergefreiter = Mladshii Serzhant (VVS) = Corporal (USAAF, RAF)

Unteroffizier = Serzhant (VVS) = Sergeant (USAAF, RAF)

Feldwebel = Starshii Serzhant (VVS) = Flight Sergeant (RAF) = no equivalent rank? (USAAF)

Oberfeldwebel = Starshina (VVS) = Warrant Officer (RAF) = Master Sergeant (USAAF)

Oberfähnrich is the rank of a "pupil officer" (an Aspirant in the Armée de l'Air, or a Zászlós in Hungary if my table is right). There was no equivalent in VVS, RAF or USA but the rank immediatly above in Luftwaffe is Leutnant (= Leitenant (VVS) = 2nd Lieutenant (USAAF) = Pilot Officer (RAF)) and there are in VVS and USAAF ranks just under the above:
Mladshii Leitenant (VVS) and Flight Officer (USA).

But they are not equivalent of Oberfähnrich, because this German rank implies the idea of someone still "training" in an officer's school. It is possible that the USN rank of Ensign implies the same thing, and so will be a good traduction.

Hope this helps

Laurent