Position an oven rack in the second-from-top slot and preheat the oven to 400°F. Line a baking sheet with foil and brush the foil with the 2 tablespoons olive oil.
Trim any excess fat from the 2 chicken breasts. Slice each breast horizontally to make 4 cutlets (cut through as if butterflying, but cut all the way through). Place the cutlets in a single layer in a gallon-size zip-top bag (or between two sheets of plastic wrap).
Pound the cutlets to an even thickness (about 1/2-inch) using a meat mallet or rolling pin. Pat the cutlets dry with paper towels.
In a shallow dish, combine 1 cup panko bread crumbs, 1/2 cup freshly grated Parmesan cheese, 1 teaspoon dried oregano, 1/2 teaspoon garlic powder, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper.
In a separate shallow dish, whisk the 1 egg until blended.
Working with one cutlet at a time, dip it into the beaten egg, letting excess drip off, then press it into the panko mixture so it is evenly coated. Place the coated cutlet on the prepared baking sheet. Repeat until all 4 cutlets are breaded.
Bake on the second-from-top rack at 400°F for 15 minutes, or until the chicken is cooked through (internal temperature reaches 165°F).
Remove the baking sheet from the oven and flip each cutlet. Spoon 1/4 cup marinara sauce over each cutlet (using all of the 1 cup marinara total), then divide the 1 1/2 cups shredded mozzarella evenly over the cutlets.
Turn the oven broiler on and return the baking sheet to the same rack. Broil 2–4 minutes, watching closely, until the cheese is melted, bubbly, and beginning to brown.
Remove from the oven and let rest 2–3 minutes. Sprinkle with the 2 tablespoons minced parsley, if using, and serve.