Quantity for over night oats

Steel cut oats take a lot longer to cook than rolled or quick oats. It's unlikely they will completely cook before the cookie is done. You could try it, but I'd recommend presoaking them in hot water for an hour, or maybe even parboiling them for a few minutes. You'd need to adjust your wet ingredients accordingly as well.

Specifically, steel cut oats look like little pellets, whereas rolled oats have the familiar flat shape from going through a steel roller. The rolled ones expose more surface area and therefore cook faster, but produce a different final texture. I do like the steel-cut ones better, but they take upwards of half an hour to cook. You can make a large batch and reheat it for 2-3 days if you like. I think some folks also use a pressure cooker to speed them up.