In a medium bowl, whisk the dry ingredients: flour, ground cinnamon, ground nutmeg, baking soda, and salt. Set aside.
Using a mixer on high speed, cream the softened butter with the brown sugar and granulated sugar until light and fluffy, about 2 to 3 minutes, scraping the sides of the bowl if needed. On medium speed, beat in the eggs, one at a time, and add the vanilla extract until evenly combined. Gradually mix in the dry ingredients until no pockets of flour remain. Do not overmix.
Mix in the oats and the raisins on low speed or using a rubber spatula. The dough should be very sticky. Cover with plastic wrap, making sure the plastic wrap is touching the dough. Refrigerate for a maximum of 30 minutes (if the dough is too cold, the cookies won’t expand properly).
Preheat oven to 350 degrees F while the dough is chilling. Line two large sheet pans with parchment paper.
After the dough has refrigerated for 30 minutes, use a cookie scoop (1.5 tablespoon capacity) to scoop out the dough onto the prepared sheet pans. Make sure the cookie dough balls are about 2 to 3 inches apart. You may need to bake them in multiple batches. Note: I'd highly recommend using a cookie scoop because the dough will be very sticky.
Bake for 11-13 minutes, or until they are slightly golden brown around the edges. Remove them from the oven and let them sit on the sheet pans for 10 minutes. Then transfer to a cooling rack and let them cool. Enjoy! Note: These cookies keep well for 5 days at room temperature if you store them airtight containers.