I've heard opinions from those whose opinions I would respect that the 'oy' is from Hebrew.

Hebrew אי לא ('y l'), (which as far as I can tell was vocalized /ʔi:lo:ʔ/), a hapax legomenon occurring at Ecclesiastes 4:10, is cited by some as the origin of Yiddish אױ (oy). The problem with that is the Yiddish is spelled in Yiddish orthography, whereas words of Hebrew/Aramaic origin are spelled in Hebrew orthography. Yiddish oy does correspond to German au (German Frau, Yiddish פרױ froy 'woman'), but it also corresponds to German o (German groß, Yiddish גרױס groys 'big') and eu (German Freude, Yiddish פרױד froyd 'joy' and the good doctor's name). I think it might be a folk etymology, but I'll ask some of my linguist buddies who speak Hebrew.

Ceci n'est pas un seing.