Heat oven to 375 degrees F. Grease or line a 13x9 inch baking pan.
Combine oats, flour, sugar, baking soda, and salt in a large bowl. Stir in melted butter until combined.
Remove 1-1/2 cups of the crumb mixture and set aside.
Press remaining crumb mixture into prepared baking pan. Bake for 12 minutes.
Meanwhile, mix sweetened condensed milk and the peanut butter in a small bowl or glass measure. Pour over partially baked crust, leaving 1/4-inch around the edges. TIP: if thick you can make more pourable by adding a splash of milk or water.
Sprinkle reserved crumb mixture and chocolate chips evenly over the milk layer, pressing down slightly.
Bake 20-22 minutes more or until golden brown around the edges.
Cool completely before cutting bars.
Notes
*Variations:
Omit chips completely.
Use any type of chip as a substitute.
Use equal amount of candy coated chocolate pieces (fun for holidays).
Substitute chopped nuts for the chips.
Storage: Keeps at room temperature in an airtight container for a couple days or in the fridge for 4-5 days.To freeze: layer the bars in an airtight container or freezer baggie, separating rows with parchment paper if needed for 3-4 months.