1. Preheat oven to 350ºF. Lightly butter a 9-inch pie plate. Line a large, rimmed baking sheet with foil. Make topping: In a bowl, combine oats, flour, brown sugar, cinnamon and salt. Stir in butter until crumbly. Toss in nuts.
2. Make filling: Bring a large pot of water to a boil; have ready a large bowl filled with ice water. Using a sharp knife, cut a shallow X into bottom of peaches. Blanch peaches until skins begin to wrinkle, 30 to 60 seconds. Remove with a slotted spoon and place in ice water to cool. Slip off skins with your fingers; cut in half, remove pits and slice fruit into wedges.
3. In a bowl, toss peaches with sugar, flour and salt. Fold in berries. Let stand for 5 minutes. Transfer to prepared pie plate. Sprinkle with topping, place on baking sheet and bake until filling bubbles and topping is golden, 50 to 55 minutes. Let cool on a wire rack. Serve warm or at room temperature, with vanilla ice cream, if desired.