Preheat oven to 350 degrees F. Line an 8-inch square baking pan with two sheets of foil placed perpendicular to one another, and long enough to overhang pan edges. Spray foil-lined pan with nonstick cooking spray, or grease with butter or shortening. (Overhanging foil will serve as handles to lift the cooked bar cookies from the pan.)

In a large bowl, combine brown sugar, flour, baking soda, salt and rolled oats, mixing well. Cut in butter using a pastry blender to form a crumbly mixture, or using a mixer, beat at low speed, scraping bowl often, until mixture is crumbly, 2 to 3 minutes. Press 1/2 of crumb mixture into bottom of prepared pan. Spread jam to within 1/4-inch of pan edge. Sprinkle remaining crumb mixture over the top, and lightly press it into jam.

Bake until topping is deep golden brown and filling is bubbling, 35 to 40 minutes. Cool to room temperature in baking pan on wire rack. Once cooled, remove from baking pan by lifting foil extensions, cut into bars and serve.

Variation: Use other types of jam, such as apricot preserves or even a bitter-orange marmalade.