Preheat oven to 325 degrees F. Pour oats into a mixing bowl, set aside.
In a microwave-safe mixing bowl (or it can also be melted in a saucepan), combine peanut butter, honey and brown sugar and heat mixture in microwave until runny and smooth, about 25 seconds. Pour mixture over oats in mixing bowl and toss mixture with a spatula until evenly coated.
Spread mixture onto a Silpat or parchment paper-lined baking sheet into an even layer and bake in preheated oven until golden brown, about 18 - 20 minutes, stirring once halfway through baking.
Allow to cool completely (it won't be crunchy until it cools) and store in an airtight container.
Notes
If you want one more layer of flavor, you can add 1/2 tsp of cinnamon.