Cream the butter and both sugars together in a large bowl until light and fluffy, about 2 minutes with an electric mixer on medium speed.
Beat in the eggs one at a time, then add the vanilla. The mixture should look smooth and slightly pale.
In a separate bowl, whisk together the flour, baking soda, salt, cinnamon, and nutmeg if using.
Gradually mix the dry ingredients into the butter mixture on low speed until just combined — don't overmix or your cookies will be tough.
Stir in the oats and raisins with a wooden spoon or spatula. The dough should look thick and speckled, not sticky.
Shape the dough into two logs about 2 inches wide on parchment paper. Roll them up tightly like jelly rolls, then refrigerate for at least 30 minutes (up to overnight works too).
Preheat your oven to 350°F and line baking sheets with parchment paper or silicone mats.
Slice the chilled dough into disks about 1/4 inch thick — this is where that chilling pays off, as the slices will hold their shape cleanly.
Space the cookies 2 inches apart on the prepared baking sheets. They'll spread slightly but shouldn't merge together.
Bake for 15-18 minutes until the edges are golden and the centers still look just barely set — they should feel firm to a gentle touch, not soft or jiggly.
