Heat oven to 375°F. In large bowl, combine plums, sugar, 1/4 cup flour and cinnamon; toss to mix. Spoon into ungreased 8-inch square pan.
In medium bowl, combine 1/3 cup flour, oats and brown sugar; mix well. Using fork or pastry blender, cut in margarine until mixture resembles coarse crumbs. Sprinkle evenly over plum mixture.
Bake at 375°F. for 35 to 45 minutes or until golden brown. Serve warm or cool.