Place the flour, oats, brown sugar, and cinnamon in a mixing bowl and rub together with your fingertips. to blend and break up any clumps of brown sugar Scatter the butter cubes over the flour mixture. Continue to rub the ingredients together with your fingertips until it forms small clumps and the butter is no longer visible. Add the walnuts and stir them in. Use immediately or store airtight in the refrigerator for as long as 3 weeks.