Combine flour, oats, brown sugar, and salt. Cut butter into it using a pastry cutter or two knives. Stir in pecans. Sprinkle on top of the peach pie filling. Place pie pan on a baking sheet lined with foil to catch drips.
Bake at 400F for 30 minutes, then cover the edges of the crust with a ring of aluminum foil and bake at 350F for another hour. (This is different from how I baked it; I baked it at 425F for 45 minutes; peaches were cooked but the pecans were burned. Covering the whole pie after 30 minutes may work to keep pecans from burning, though.) Cool completely, or at least 90 minutes, before cutting.
Filling adapted from the Betty Crocker Cookbook; streusel adapted from Bon Appetit