While Spanish speakers *do* use "Ay, Dios mío" (pronounced "eye Dee-ohss mee-oh") as an exclamation of surprise, amazement, or dismay, it's not a direct translation of "OMG."
"Ay, Dios mío" means "Oh, my God", which is a bit more formal and reverent than "OMG."
Spanish speakers do have a more casual equivalent of "OMG," which is "¡Ay, caramba!" (pronounced "eye car-am-bah") or "¡Ay, madre!" (pronounced "eye mah-dray"). These are both interjections expressing surprise or annoyance, similar to "Oh my goodness!" or "Oh, man!"
So, while "Ay, Dios mío" is a valid Spanish expression, it's not the exact equivalent of "OMG."