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Pull them while the centres still look wet
Chew in this cookie is decided by when it leaves the oven, not by anything you can add to the bowl.
Oats keep absorbing and firming for as long as the cookie is hot, so a tray pulled at the point most people call done carries on past done as it cools, and dry is the opposite of chewy. Take them out with the edges set and the middles still glossy, and let fifteen minutes on the hot sheet finish them.
Why this and not an ingredient: nothing in the formula fitted as a chew lever for this dish once we refitted it. The bake did.
Goes the other way
129 bakes
Brown sugar makes them thicker, not chewier
Cooks who pushed the brown-to-white ratio up got a taller, less spread cookie, and no measurable change in chew.
That is worth knowing because the advice you will be given is the reverse: brown sugar is recommended everywhere as the chew lever. In this dough it is a THICKNESS lever. If you want a cookie that holds its height, this is a real tool. If you want chew, it is the wrong dial and the oven is the right one.
From 129 bakes where a cook moved the sugar ratio and told us what came out.
Not for chew
Quick oats make them softer, not chewier
This is the other half of the advice you will be given, and it is aimed at the wrong outcome.
Quick oats are cut smaller and rolled thinner, so they drink water faster and vanish further into the dough. Cooks who swapped them in reported a measurably softer edge and a smoother cookie, with nobody reporting the opposite. Soft and chewy are not the same texture: chew is the crumb pushing back, and a quick oat gives you less to push against. Use rolled oats if chew is the goal.
Which one you want: the oats page walks the whole ladder, from steel-cut down to oat flour.