In a medium saucepan, whisk together the orange juice, cornstarch and 2 tablespoons of the brown sugar. Add the raspberries and peaches, then stir over medium-high until the mixture is simmering and thickened, about 5 minutes. Remove from the heat and set aside to cool. Heat the oven to 350F. Coat a 7-by-11 inch (or 9-by-9 inch) baking dish with cooking spray. In a large mixing bowl, combine the oats, flour, remaining 1/2 cup of brown sugar, butter, and baking soda. Mix together with a fork until the dough is crumbly but beginning to hold together. Set aside 1 cup of the mixture. Mix the egg into the dough remaining in the bowl. Transfer the dough with the egg to the prepared baking dish and press it evenly over the bottom. Spread the reserved fruit filling on top, then sprinkle the reserved crumb mixture over it. Bake for 30 to 35 minutes, or until lightly browned and bubbling. Transfer the dish to a wire rack to cool completely, about 1 hour. Cut into 16 bars.