Butter should be cool to the touch and slightly soft, but not squishy. Your finger should leave an indent, but not press through the butter.
Even though this is a no-chill recipe, a short rest, even 15–30 minutes, lets the oats hydrate, and the flour fully absorb moisture for softer, thicker cookies.
Always start with a cool baking sheet. Putting dough onto a warm pan can cause cookies to spread before they even hit the oven.
Every oven runs a bit differently. For chewy cookies with soft centers, start checking at 8 minutes. For crispy cookies, bake for a minute or two longer.