Preheat oven to 350°F. Grease four 6-oz ramekins generously and set on a baking sheet.
Blend rolled oats into oat flour if starting from whole oats. Whisk oat flour, protein powder, baking powder, baking soda, cinnamon, and salt in a bowl.
Whisk eggs, Greek yogurt, milk, maple syrup, and vanilla together in a separate bowl until smooth.
Pour wet ingredients into dry. Stir until just combined. Do not overmix. Fold in mix-ins with 2 to 3 strokes.
Divide batter evenly among ramekins, filling three-quarters full. Press extra mix-ins onto the surface. Bake 18 to 22 minutes until tops are set and golden and a toothpick comes out with moist crumbs.
Cool 5 minutes in ramekins before serving warm or transferring to a rack to cool fully for meal prep storage.