Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a large mixing bowl, combine the rolled oats, milk, peanut butter, honey or maple syrup, protein powder, vanilla extract, baking powder, and salt. Stir until well combined.
If desired, fold in chocolate chips, nuts, or dried fruit for added flavor and texture.
Divide the mixture evenly among the muffin cups, filling each about 3/4 full.
Bake in the preheated oven for 15-20 minutes, or until the tops are golden and a toothpick inserted in the center comes out clean.
Allow the oatmeal cups to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.