Arrange the pineapple rings and cherries on the bottom of a buttered pie pan. You will likely have a few pineapple rings left—cut these up into small pieces to add to the batter.
In a large bowl, stir together the oats, sugar, baking powder, and salt.
In a smaller bowl, stir together the milk, egg, melted and cooled butter, and vanilla extract.
Combine the wet ingredient with the dry in the large bowl and stir to combine. Then, stir in the chopped pineapple pieces.
Add this mixture to a pie pan, add your pineapple and cherry pieces, and cover and store overnight in the refrigerator.
In the morning, remove the cover and bake at 350°F for 30-35 minutes. If you want to be fancy, you can turn the slightly cooled pan out onto a cutting board so you can see the pineapple slices. But it's not necessary—just a fun presentation.