Preheat oven to 350 degrees F.
To prepare topping: Combine oats, walnuts, brown sugar, flour and cinnamon in a medium bowl. Drizzle with oil and stir until evenly moist.
To prepare filling: Combine pears, maple syrup, raisins, flour, lemon juice and ginger in a large bowl and mix well. Transfer the mixture to a 9-by-13-inch baking dish. Sprinkle the topping over the pears.
Bake the crumble until the pears are tender and the topping is golden, 45 to 50 minutes. Let stand for at least 10 minutes before serving.
Make Ahead Tip: Prepare the topping (Step 2) and filling (Step 3), cover and refrigerate separately for up to 1 day. Bring to room temperature before assembling and baking.
1 1/2 fruit, 2 other carbohydrates, 2 fat