Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, sugar, oats, and 1/4 teaspoon cinnamon in a medium bowl; cut in butter with a pastry blender or 2 knives until mixture is crumbly. Stir in walnuts.
Combine apple and remaining ingredients in a large bowl; toss well. Spoon apple mixture into an 8-inch square baking dish or 1 1/2-quart casserole. Sprinkle with crumb mixture. Bake at 375° for 45 minutes or until golden brown. Serve warm.