Combine salt, onion powder, garlic powder, oregano, cayenne, and pepper in small bowl.

Toss potato wedges with 4 teaspoons spice mixture and oil in large bowl; cover. Microwave until potatoes are tender but not falling apart, 7 to 9 minutes, shaking bowl to redistribute potatoes halfway through cooking. Uncover and drain potatoes. Arrange potatoes on rimmed baking sheet and let cool until potatoes firm up, about 10 minutes.

Set wire rack in rimmed baking sheet and line second baking sheet with triple layer of paper towels. Add remaining peanut oil to large pot and heat over medium-high heat to 340°F. Meanwhile, combine flour and cornstarch in medium bowl and whisk buttermilk and baking soda together in large bowl. Working in 2 batches, dredge potato wedges in flour mixture, shaking off excess. Dip in buttermilk mixture, allowing excess to drip back into bowl, then coat again in flour mixture. Shake off excess and place on wire rack.

When oil is ready, add half of coated wedges and fry until deep golden brown, 4 to 6 minutes. Transfer wedges to large bowl and toss with 1 teaspoon spice mixture. Drain wedges on paper towel lined baking sheet. Return oil to 340°F and repeat with a second batch of wedges. Serve with extra seasoning on the side