Blend first 5 ingredients in processor. Add butter; using on/off turns, blend until moist dough forms (mixture will resemble wet sand). Add oats; using on/off turns, mix briefly, leaving half of oats whole. Set aside.
If pastry has been chilled for a long time, let it sit at room temperature for at least 15 minutes before rolling.
Preheat oven to 375 F.
Lightly flour a pastry board, marble counter, or kitchen counter. Pat one pastry disc (save other portion for another pie)into a flat round. Lightly flour the rolling pin. Roll pastry in one direction only, turning pastry continually to prevent it from sticking to the surface.
Using pie plate as a guide, measure rolled-out pastry — it should be slightly larger than the pie plate and ⅛-inch thick. Fold rolled pastry circle in half so you can lift it more easily. Unfold, gently fitting the pastry into the pie plate. There will be some of the pastry hanging over. You could trim a little around with scissors and leave enough to crimp.
Fill with your apple filling.
Sprinkle streusel topping evenly over filling.
Place pie on baking sheet.
Bake pie until crust is golden brown and juices bubble, 50 minutes–1 hour. *My pie was ready at 45 minutes. Every oven is different. I didn't want my topping to get too brown.
Transfer to a wire rack to cool.