Combine the salt,pepper,onion and garlic powders,cayenne, and oregano in a small bowl.
Toss the potato wedges with 4 teaspoons of spice mixture and 1/4 cup oil in a large microwave-safe bowl; cover tightly with plastic wrap. microwave on high until potatoes are tender but not falling apart, 7-9 minutes, shaking the bowl (without removing the plastic) to redistribute the potatoes halfway through cooking. slowly remove the plastic wrap from the bowl (be careful of the steam) and drain the potatoes. arrange the potatoes on a rimmed baking sheet and cool until they firm up, about 10 minutes. (potatoes can stay at room temp up to 2 hrs if need be) heat the remaining 3 quarts of oil in a large dutch oven over high heat to 340 degrees.meanwhile, combine the flour,cornstarch in a medium bowl and whisk the buttermilk and baking soda in a large bowl. working in 2 batches, dredge the potato wedges in the flour mixture shaking off excess, dip in the buttermilk mixture,allowing the excess to drip.back into bowl, then coat again in the flour mixture. shake off the excess and place on a wire rack,(the potatoes can be coated up to 30 min in advance).
when the oil is ready,add half the coated wedges and fry until deep golden brown, 4 to 6 minutes. transfer the wedges to a large bowl and toss with 1 teaspoon of the spice mixture. drain the wedges on a baking sheet lined with paper towels. return the oil to 340 degrees and repeat with the second batch of wedges. serve with extra spice mixture on the side.
can serve with creamy horseradish sauce or other sauces.