Cream together brown sugar and margarine until light and fluffy (around 1 minute on high speed).
To the creamed ingredients, add oats, flour, baking powder, baking soda, and salt. Mix on low speed to combine.
Transfer two-thirds of the mixture to a square 9-inch baking pan. Gently press it into an even layer (but don't press too firmly; we don't want the cake to be dense).
Spread the rhubarb jam over top of the oat mixture. Make sure to cover the entire surface but don't stress if there is more jam in some areas than others.
Sprinkle the remaining oat mixture over to lightly cover the jam.
Bake for 40 minutes at 325 °F. Let cool fully before slicing for the best flavour and texture. Enjoy!