Preheat the oven to 450 degrees. In a bowl, mix the calamari and buttermilk. In another bowl, whisk the cornmeal, garlic powder, cayenne, 3/4 tsp. salt and 1/2 tsp. pepper. Working with a few pieces at a time, toss the calamari in the cornmeal mixture, shaking off excess. Arrange on a large parchment-lined baking sheet. Coat with cooking spray. Bake until tender and golden brown, about 18 minutes. Transfer to a serving dish; sprinkle with the oregano. Serve immediately with the lemon wedges and marinara sauce.