Let me provide some background here. Many localities in Poland have both German and Polish names, due to the fact that western Poland was under a German jurisdiction for extended period of time (between 1772? and 1918). The use of the German or the Polish name is often a political statement, and became even more so after WW2.
The post-war government made a concerted propaganda effort to legitimize the annexation of the so-called "recouped territories" (Silesia, Pomerania, Ost Preussen, etc.) as a result of the Yalta agreement. This involved displacement of much of the German population inhabiting these territories to make room for Polish expats displaced from Eastern Poland, annexed by the USSR. The insistence on using Polish names instead of German ones was a part of this propaganda effort.
However, Poles being Poles, they often tried to spite the government by mocking national pieties and idols. Hence their deliberate usage of German names in certain occasions. I recall once being asked by a German tourist how to get to Gdynia, which is a port city built by Poles in the 1920s to bypass the German-controlled port of Gdansk, but had also the German name Gottenhafen. I gave him directions using the German name and saw his jaw dropping.
So I am pretty sure that the Polish person Justin mentions was either using the German term in the sense described above, or perhaps "translated" it because German names are often used instead of difficult to pronounce Polish names in many western countries. That was certainly the case of Germany, where Polish visitors or "guest workers" would most likely used the German names while talking to Germans.
I cannot think of any other circumstances when somebody would use "Auschwitz" instead of "Oswiecim." Btw, Oswiecim is a relatively large city in a highly industrialized region which would like to be known for things other than the concentration camp. I am pretty sure the locals must be pissed by tourists who see only the camp and do not give two shits about their city. Not that I think that two shits are due, but then I am not local.
Wojtek