Heat Oven. Preheat oven to 350 degrees F. Grease a 9 x13-inch baking dish with nonstick cooking spray.
Make Dough. Mix together melted butter and brown sugar in a large mixing bowl until smooth. Add eggs and vanilla extract. Mix until thick. Add flour, baking soda, cinnamon, and salt. Mix until smooth. Add the oats and chocolate chips. Mix until combined.
Bake. Press dough into prepared baking dish. The dough will be thick. To make this easier, wet your hands with cool water. This prevents the dough from sticking to your hands.
Bake until bars are golden brown and set, about 30 minutes.
Cool and Cut. Cool bars in the pan. Cut into squares. Store bars wrapped on the counter for up to four days. Freeze cooled bars for up to three months.
Brown Sugar: Light or dark brown sugar work well in these bars. Dark brown sugar adds more molasses flavor and makes the bars slightly softer than those made with light brown sugar.
Freezing Directions: Allow the bars to cool. Cut into squares. Place the bars in even layers in a freezer container. Place a piece of parchment paper between the layers. Thaw the bars overnight on the counter.