Heat oven to 350°F. Spray 13x9-inch pan with cooking spray; lightly flour. In large bowl, beat 1/2 cup butter, 1 cup brown sugar and the granulated sugar with electric mixer on medium speed until creamy. Beat in eggs and vanilla. On low speed, beat in flour mixture just until blended. Beat in oatmeal. Pour into pan. Bake 30 to 35 minutes or until toothpick inserted in center comes out clean.