Clean the mushrooms and remove any long stems, which can be saved for another use or discarded. Ideally, each mushroom will be 1 inch wide or less, and you may want to cut in half any that are larger so they’ll be easier to eat.
In a large food storage bag, add the flour. Add the prepared mushrooms and shake until well-coated.
Pour the buttermilk into a mixing bowl. In another mixing bowl, add the panko breadcrumbs on a plate then stir in the garlic powder, chili powder, oregano, salt, and pepper.
Dip the flour-coated mushrooms into the buttermilk then lightly dredge in the panko. Place the coated mushrooms on a large plate or sheet. Repeat until all are done.
In a large heavy skillet heat 1/2 inch of oil on medium-high to 350°F, about 5 minutes. If you don’t have a thermometer, you can test the temperature by sticking a wooden spoon into the oil. If it bubbles around the spoon, it should be ready for frying.
Line a large plate or sheet with paper towels.
Working in batches, slide the mushrooms into the hot oil and cook for 30 seconds then turn and cook for another minute or until golden and crisp. Remove with a slotted spatula to drain on the paper-lined plate.