Adjust the oven rack so the upper and lower middle positions and heat the oven to 350°. Line 2 large baking sheets with parchment paper or spray them with nonstick cooking spray.
Whisk the flour, baking powder, nutmeg and salt together in a medium bowl.
Either by hand or with an electric mixer, beat the butter on medium speed until creamy. Add the sugars; beat until fluffy, about 3 minutes. Beat in the eggs, 1 at a time.
Stir the dry ingredients into the butter-sugar mixture with a wooden spoon or large rubber spatula. Stir in the oats and raisins (if using).
Working with a heaping 2 T each time, roll the dough into 2-inch balls. Place the balls on the prepared baking sheets, spacing them 2½ inches apart.
Bake until the cookie edges barely turn golden brown, 21-24 minutes, rotating the baking sheets front to back and top to bottom halfway through the baking time. They continue to cook out of the oven so make sure to get them out a bit early. Let the cookies cool on the baking sheet for 2 minutes. Transfer the cookies with a wide metal spatula to a wire rack. Let cool at least 30 minutes. (I chose to not wait 30 minutes and may have burned my mouth.)
Recipe by The Baker Chick at https://www.thebakerchick.com/classic-oatmeal-cookies-with-chocolate-chips/