Heat the oven to 375 degrees F. Spread oats in a single layer on half-sheet pans and bake until lightly toasted, about 20 minutes. Remove the oats from the oven and let cool for 2 to 3 minutes.
Grind 8 ounces of the toasted oats in a food processor for 3 minutes or until the consistency of whole-wheat flour. Add the baking powder, cinnamon and salt and pulse to combine.
Combine the butter, brown sugar and granulated sugar in the bowl of a stand mixer, and mix on medium speed until light in color, about 3 minutes. Stop once to scrape down the sides of the bowl. Reduce the mixer speed to the lowest speed, add the egg and vanilla and mix to combine. Stop to scrape down the sides of the bowl, if necessary.
With the mixer on the lowest speed, slowly add the oat mixture, the remaining 8 ounces of oats and the raisins until just combined. Stop once to scrape down the sides of the bowl. Scoop the dough with a 1-ounce ice cream scoop or disher onto parchment-lined half-sheet pans, leaving 2 inches between each mound. Bake the cookies for 12 minutes, rotating the pans after 6 minutes, until the cookies begin to brown around the edges. Remove the pans from the oven and let the cookies cool on the pans for 2 minutes. Transfer the cookies to a cooling rack to cool completely.