Preheat oven to 350°F. Arrange mason jars on a rimmed baking sheet.
Combine raspberries, blackberries, dark brown sugar, cornstarch, lemon juice, almond extract and salt in a large bowl. Divide evenly among mason jars.
Mix oats, sugar, flour, cinnamon and nutmeg in a large bowl. Add butter and work into dry ingredients until mixture is crumbly. Sprinkle oat mixture evenly over berries in each mason jar. Bake 40 minutes or until berries are bubbly and topping is lightly browned and crisp. Serve warm or room temperature.