Preheat oven to 400F.
In a large mixing bowl combine berries, sugar, flour, cornstarch, lemon juice, (optional) lemon zest, and vanilla.
Toss to combine, until evenly coated. Chill in the refrigerator until needed.
In a medium mixing bowl, combine oats, brown sugar, flour, cinnamon, and salt. Using a pastry cutter (or a fork), cut in butter until pea-size clumps form. You can also just pulse the whole thing in a food processor. Place in refrigerator until needed.
Prep your pie crust as per package instructions (it might require pre-baking) or if using homemade crust, roll it into a 9-inch pie plate. Press gently into the bottom of the pan. Flute the edge, or crimp using a fork.
Add pie filling, then sprinkle the oat flour mixture on top of the pie filling. Bake for 30 minutes.
Cover with foil, and continue baking for 20-25 minutes. Tent the foil at the 45-minute mark to prevent the crust and topping from getting too brown.
Remove from the oven and allow the pie to cool before serving. Refrigerating helps set it beautifully.