Mix brown sugar, oil, vanilla and egg together.
In a separate bowl, combine the flour, oatmeal, salt and baking soda.
Combine the two mixtures.
Separate the dough in half and pack one half of the dough into a greased 9X13 pan.
Sprinkle chocolate chips over the dough and then sprinkle the marshmallows on top.
Crumble the remaining dough on top and bake at 350 for 12-15 minutes.
I think these taste better warm, but they are a little bit crumbly at that point so you may need to eat them with a spoon! Enjoy!