Step 1: Preheat oven to 350°F (175°C) and line an 8×8-inch baking pan with parchment paper.
Step 2: In a large bowl, combine oats, flour, brown sugar, cinnamon, baking powder, and salt.
Step 3: In another bowl, whisk together melted butter, honey, and eggs until smooth.
Step 4: Pour the wet ingredients into the dry ingredients and mix until just combined.
Step 5: Fold in the diced apples and walnuts (if using) until evenly distributed.
Step 6: Spread the batter evenly into the prepared pan.
Step 7: Bake for 25-30 minutes, or until golden brown and a toothpick inserted comes out clean.
Step 8: Let cool completely in the pan before cutting into 12 bars.