Preheat oven to 350 degrees F (175 degrees C).

Stir oats, milk chocolate pieces, flour, flax seed meal, brown sugar, and salt together in a large bowl. Make a well in the center of the oat mixture.

Whisk egg in a separate bowl until light and fluffy. Add honey, oil, and vanilla; whisk until well-combined. Pour egg mixture into the well in the oat mixture. Stir until evenly combined. Pour mixture into a 9x13-inch glass baking dish; pat down and spread the mixture flat, leaving at least 1-inch of space between the mixture and the sides of the dish.

Bake in the preheated oven until golden and set, about 25 minutes.