Preheat the oven to 350 degrees. Grease a 9 inch square baking pan.
In a small sauce pot, combine the cranberry sauce with ¼ cup white sugar. Heat at medium heat for a couple of minutes, while stirring, until the sugar is incorporate and completely dissolved. (You can also use your microwave for this in a microwave safe bowl)
In a medium bowl, combine the flour, oats, brown sugar, & salt. Using your fingers, work the 1 stick of softened butter into the dry ingredients until well mixed & crumbly. Stir in walnuts if using (optional).
Firmly press 2 cups of the streusel topping into the bottom of your greased baking dish. Spread the cranberry sauce mixture over the crust. Top with the remaining streusel.
Bake for 30 minutes, or until the crust is lightly browned. Let cool completely before slicing
For the cranberry sauce (optional - if you don't have leftover)
Combine the sugars & orange juice in a small sauce pan. Cook on medium until the sugar is dissolved. Add in the cranberries, and cook for 10 minutes - stirring occasionally. Sauce will thicken as it cools.