Preheat oven to 350°F.
Peel, core, and slice apples. Toss with lemon juice and place in buttered pan.
Combine sugar, flour, oats, and cinnamon in the bowl. Melt the butter and add it to the mixture, stirring with a fork or pastry blender until it is well blended. Add nuts.
Sprinkle topping evenly over apples and bake 45-50 minutes or until apples are tender and it bubbles around the edges.