How to Make It
Combine cranberries, sugar, marmalade and zest in a small pan. Add 2 Tbsp. water; bring to a boil, stirring and slightly crushing berries. Reduce heat to medium-high; cook until reduced to 1 cup, 5 to 10 minutes, stirring often. Transfer to a bowl, partially cover and refrigerate until thoroughly chilled, at least 2 hours. (May be made up to 3 days in advance; keep covered and chilled.)
Preheat oven to 350°F. Coat a 9-inch square baking pan with cooking spray. Line with foil, allowing an overhang of 2 inches; spray foil.
Combine oats, flour, brown sugar, baking powder and salt in a bowl. Use your fingers or a pastry blender to work butter into mixture until butter is the size of small peas.
Press 1/2 of oat mixture in bottom of pan. Spread cranberry mixture evenly over crust. Crumble remaining oat mixture on top; pack down slightly. Bake until top and edges are lightly browned, approximately 30 minutes. Allow to cool completely before cutting.