Preheat the oven to 350F
In a large bowl, cream together the butter and sugar until smooth. Add in the egg and vanilla extract, beating until light and fluffy.
In a separate bowl, whisk together flour, baking soda, cinnamon, and salt.
Gradually mix the dry ingredients into the butter mixture until just combined. Fold in the old-fashioned oats.
Divide the dough into 10 even portions (about 39 g each) and roll into balls.
Line a baking sheet with a silicone baking mat or parchment paper, and space out the dough balls evenly (about 6 cookies per half sheet).
Bake for 15 minutes, rotating the pan halfway through, until the edges are lightly golden.
Let cool completely on the tray for crispier cookies. Enjoy!