Preheat oven to 375 degrees F. Stir oats, milk, pumpkin, brown sugar, vanilla, baking powder, pumpkin pie spice, salt and eggs together in a large bowl until fully incorporated.
Lightly coat a 12-cup muffin tin with cooking spray. Spoon the batter into the prepared muffin cups, filling each almost to the top. Sprinkle evenly with pecans.
Bake the muffins until a toothpick inserted in the center comes out clean, 25 to 30 minutes. Let cool in the pan for 10 minutes, then transfer to wire rack. Serve warm or at room temperature.
To make ahead: Wrap airtight and refrigerate for up to 2 days or freeze for up to 3 months.