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!