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318 bakes
Push the brown-to-white sugar ratio up
Of everything people suggest for a chewier oatmeal cookie, this is the one our data actually supports, and it is the best-evidenced lever on the dish.
Brown sugar carries molasses, molasses holds on to water, and water still in the crumb after baking is what chew physically is. Going all-brown is the strongest version of the change. It is also the most common thing cooks adjust here, which is why we can see it so clearly.
From 318 bakes where a cook moved the sugar ratio and told us what came out.
The cost
46 bakes
Expect a wider cookie, and stop flattening it
The same brown sugar that buys you chew is this dough's strongest push toward a flatter, wider cookie.
That is a real trade and it is the reason people chase chew and end up with a puddle. The compensation is free: leave the dough in tall rough balls instead of pressing them down. Cooks who did that kept the spread in check, and it costs nothing in the bowl. If yours are still going wide, the spread page works through the rest in order.
From 46 bakes. Fewer than the chew result rests on, so treat the direction as solid and the size as approximate.
Also holds
318 bakes
Do not cut the total sugar
Sugar holds water too, so taking it out costs you chew alongside sweetness.
The effect is real and much smaller than the brown-to-white ratio, so this is a thing to avoid doing rather than a lever to pull. If you want it less sweet without losing the texture, that is a flavour problem to solve with salt rather than by cutting the sugar.
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.