Line a baking sheet with paper towels and place a wire rack on top.
Place 1 cup flour in a bowl, pour in the buttermilk and whisk to combine until no lumps remain. In a second bowl, combine the remaining flour, cornstarch, oregano, salt, garlic powder, onion powder, paprika and pepper and whisk to combine.
Heat the oil to 350°F in a heavy-bottomed pot over medium-high heat. Coat the mushrooms in the dry flour mixture and then dip them in the buttermilk mixture, ensuring the whole mushroom is covered.
Carefully place them back in the dry flour mixture and press it onto them to coat.
Fry the mushrooms in the hot oil for 2 minutes, then flip and fry for 2 more minutes. Remove to the prepared baking sheet and sprinkle with flaky salt.