Volume measurement (measuring cups) is extremely in accurate. Volume measurement is a measure of how much space something takes up.
6 cups flour can be any where from 720g to 900g, depending on how the cup is packed.
4 sticks of butter is 454g.
Let’s assume you stir, then scoop the flour into the cup with a spoon and level. You might be closer to 120g per cup for 720 g total.
454 ÷ 720 = .63
Your butter is at 63%. That’s slightly lower than the average of 70% for a chocolate chip cookie.
But there’s enough butter that the cookie shouldn’t be dry if you add oatmeal.
A chocolate oatmeal chipper normally has more oatmeal to flour by weight. It’s usually 33% more. The problem with your recipe is you’re using for the measurement so it makes it difficult to convert.
You’ll have to experiment, replace about 50% of the flour with rolled oats.
I would run a test patch with a smaller portion instead of the full amount. That’s the problem with volume measurement, you can’t scale anything or make any adjustments. This is why professional bakers don’t use cups and spoons.
The other alternative is to just find a oatmeal chocolate chip recipe. It might not even be worth the trouble to revise your recipe.