Preheat oven to 375 degrees F (190 degrees C). Grease a 2-quart baking dish.
Combine rhubarb, strawberries, white sugar, lemon juice, and lemon zest together in a bowl.
Stir brown sugar, whole wheat flour, oats, salt, and cinnamon together in a bowl. Cut cold butter into oat mixture with a pastry blender until the mixture resembles coarse crumbs and butter pieces are pea-sized.
Pour rhubarb mixture into prepared baking dish. Spread oatmeal mixture evenly over the top.
Bake in the preheated oven until golden and bubbling, about 35 minutes.