Preheat oven to 350°F. Line a 13x9x2-inch baking pan with foil, extending foil over edges of pan. Grease foil; set aside. In a bowl, combine flour, oats, brown sugar, baking soda, and salt. Add melted butter and mix until crumbly. Reserve half of crumb mixture (about 2 1/2 cups) for topping. Press remaining crumb mixture into bottom of prepared baking pan.