Grease a 13x9-inch pan, and line with foil or parchment paper.
Combine brown sugar, flour, baking soda, salt, and rolled oats. Cut in butter with a pastry blender or two knives until mixture is crumbly.
Press 2/3 of the crust mixture into the prepared pan. Slice the guava paste into enough pieces to cover the crust within 1/4 inch of the edge, and place in pan. Sprinkle remaining crust mixture over guava paste layer.