Combine rolled oats and milk in a medium mixing bowl. Allow to hydrate for 15 minutes to ensure a tender crumb.

Preheat convection oven to 375°F (190°C) and line a 12-cup standard muffin tin with paper liners.

In a large bowl, sift together flour, baking powder, baking soda, salt, and cinnamon. Ensure even distribution of leavening agents.

In a separate vessel, whisk together the melted butter, honey, egg, and vanilla extract until emulsified.

Incorporate the wet mixture and the soaked oat-milk mixture into the dry ingredients. Stir manually using a spatula until just combined; do not overmix to avoid gluten development.

Divide the batter into three equal portions (approximately 1.25 cups each).

Fold blueberries into the first portion, chocolate chips into the second, and chopped walnuts into the third.

Distribute the batter evenly into the prepared muffin tin, placing 4 of each variation into the cups.

Bake for 18–20 minutes, or until the internal temperature reaches 200°F (93°C) and a toothpick inserted into the center comes out clean.

Cool in the tin for 5 minutes before transferring to a wire rack for final cooling.