Prep. Preheat your oven to 350°F and line 2 baking sheets with parchment paper.
Cream. In a large mixing bowl, cream together the butter, granulated sugar, and brown sugar until fluffy and significantly lighter in color.
Add the rest of the wet ingredients. Stir in the egg followed by the egg yolk. Then mix in the vanilla. Finally, stir in the molasses. Scrape down the sides of the bowl and mix until everything is well incorporated.
Combine the dry ingredients. In a separate bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, ground cloves, ground nutmeg, ground ginger, and salt.
Put it all together. Add the dry ingredients to the wet ingredients. Mix until everything is just combined. Be careful not to over-mix. Fold in the oats. Again, careful not to over-mix.
Chill. Cover the cookie dough with plastic wrap or seal it in a Ziplock and refrigerate for at least 30 minutes (up to 24 hours).
Bake. Use a medium cookie scoop to portion the dough out into equal balls. Arrange the dough balls on the prepared cookie sheets and bake for 15 minutes.
Cool. Remove the cookies from the oven and allow them to cool on the cookie sheet for a few minutes before transferring them to a wire rack to cool completely.
Notes
While I wouldn't suggest adding any chocolate chips to this ginger molasses cookie, you could always turn them into iced gingerbread oatmeal cookies by whipping up some vanilla icing and adding a dash of maple syrup for some extra flavor and using a piping bag to drizzle it onto the warm cookie.