Add butter to a small light-colored skillet and melt over medium heat. Stir or swirl the pan frequently. (Using a light-colored pan helps to be able to see the color of the flecks in the browned butter.)
Once the butter is melted it will start to foam and then subside. You will see small flecks at the bottom of the pan. When they have reached light brown and the butter smells nutty it is ready.
Browned butter can burn very flip-flop so if the flecks turn black you will have to throw it out and start over.
Once your butter is ready, take it off the heat and let it slightly cool.
In a medium-sized mixing bowl add flour, rolled oats, brown sugar, and cinnamon.
Pour the butter over the mixture and stir to combine.
The mixture will look crumbly and the mixture should be evenly incorporated.