Preheat oven to 350°F. In a mixing bowl, beat butter, brown sugar, and granulated sugar until creamy.

Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.

In a separate bowl, combine flour, oats, baking soda, and cinnamon. Gradually add to the wet mixture and stir until combined.

Press the dough into a greased 9-inch pie dish to form the crust.

Bake the crust for 15 minutes, then remove and let cool.

For the filling, mix together marshmallow creme and confectioners' sugar. Add milk a tablespoon at a time until spreadable.

Spread the marshmallow filling over the cooled crust evenly.

Chill the pie for at least 2 hours before serving to allow the filling to set.