1. Whisk together the lime juice, vinegar, sugar, and salt in a medium sized non-reactive bowl and let sit until the salt and sugar dissolve, about 5 minutes.

2. Meanwhile peel and cut the onions in half vertically, then cut into very thin slices about an  1/8 inch thick. Add the onions and the oregano to the bowl with the lime juice mixture and stir well to combine and separate the onion slices. You could add 2 whole serrano or jalapeno chiles slit down the center at this point if you want spicier pickled red onions. Cover and refrigerate for at least 24 hours and up to 2 days, stirring the mixture at least once during this time. Drain before serving.