When I was in school one of my teacher(a rebbe/rabbi) told us a long time ago everyone use to pronounce everything OH and then the Perushim came along and they were vrey makpid on a lot of things like dikduk so then we changed the havarah from oh -> oy
we wanted to show that we are nothing like the perushim so we now-the men pronounce it as Oy-it is really Oh-if you go to a yekkee shul I belive that they pronounce everything/ lot of words as OH!
The PERUSHIM?????? No. To make the sound of OY in Hebrew, all one needs to do is write something with a cholam (OH) followed by a Yud. In some cases a kawmatz and Yud will accomplsih the same thing, but that mostly applies to ashkenaz pronunciation, and the difference between the two would be OH-ee versus Aw-ee. Is it possible your rebbie said something else, not “Perushim”? The Perushim were the najority of frum Jews, those who followed both the Torah Shebichsav and the Torah sheb’al peh. they did not lead an ascetic life like the Essenes, and did not sit in the dark on Shabbos and eat cold food, like the Karaites.
I thought Yekkim pronounce a lot of Ohs as Ows. (I am really not knowledgeable about this, it is only a thought in my memory bank).