Mix pears, brown sugar, flour, lemon juice, vanilla extract, cinnamon, nutmeg, and allspice together in a bowl. Transfer the mixture to a buttered 9" pie dish.
In the same bowl used for step 1, mix together rolled oats, all-purpose flour, oat flour, brown sugar, and cinnamon. Stir in melted butter; the mixture should be damp and crumbly.