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

In a large bowl, combine oats, flour, and baking soda.

In a separate bowl, beat butter and brown sugar with a mixer until blended.

Add the oat mixture to the butter mixture and mix well.

Press 3 cups of the mixture onto the bottom of a 13x9-inch pan that has been sprayed with cooking spray to form the crust.

In a microwave-safe bowl, melt chocolate in 30-second intervals, stirring until smooth.

Stir in peanut butter until combined.

Carefully spread the peanut butter chocolate mixture over the crust.

Sprinkle the remaining crumb mixture over the chocolate layer.

Bake for 22 to 25 minutes, or until the edges are golden brown.

Let cool completely before cutting into bars.