Preheat your oven to 350 degrees. Lightly grease a 1 & 1/2 quart casserole dish.
Gently toss the yams & the cranberries together in the prepared dish.
In a medium bowl, mix together the flour, brown sugar, Oats, butter & cinnamon until well combined. Add about half of the Oat mixture to the yams & cranberries. Gently toss to combine. Sprinkle the remaining oat mixture over the top of the yam mixture.
Bake for 30 - 35 minutes or until lightly browned. Remove from the oven & sprinkle the marshmallows across the top of the dish if using. Return to the oven & bake for an additional 5 minutes or until the marshmallows are starting to turn golden & begin to expand & melt.