Preheat oven to 325°F. Line a large rimmed baking sheet with parchment paper. Microwave 1 1/2 tablespoons butter in a small microwavable bowl on High until melted, about 30 seconds. Stir together oats, chestnuts, flour, brown sugar, cinnamon, and salt in a medium bowl. Drizzle in oil and melted butter; stir until well coated. Place mixture on prepared baking sheet, and shape into an even, compact 5-inch disk. Bake until deeply browned and fragrant, about 25 minutes (crumble will not be set). Let cool on baking sheet 25 minutes (do not turn off oven). Crumble into pieces.