Preheat oven to 350°F. Line a large baking sheet with parchment paper or a silicone baking mat and set aside.
In a medium sized mixing bowl, using a hand-held mixer, beat the butter, brown sugar, and granulated sugar until light and fluffy. Add in the egg yolk and vanilla and mix until well combined, scraping down the sides of the bowl as needed.
Add in the flour, baking soda, and salt and mix until everything is fully combined. Then mix in the rolled oats and semi sweet chocolate chips until fully incorporated into the dough.
Scoop out two tablespoon sized pieces of dough onto the prepared baking sheet, you should end up with about 8-9 cookies.
Bake at 350°F for 9-11 minutes. Remove the cookies from the oven and cool on the baking sheet for 5 minutes.