Combine the oats, flour, cinnamon, baking powder and salt in a large bowl. Beat the butter, sugar and molasses in a large bowl with a mixer on medium-high speed until fluffy, about 5 minutes. Beat in the egg and vanilla until smooth, about 2 more minutes. Reduce the mixer speed to low, add the flour mixture and beat until combined. Stir in the raisins by hand. For the best flavor and texture, cover the dough and chill at least 4 hours, or overnight.
Position racks in the upper and lower thirds of the oven; preheat to 350 degrees F. Line 2 baking sheets with parchment paper. Form the dough into 12 balls, about 2 tablespoonfuls each, and arrange 3 inches apart on the prepared baking sheets. Flatten with the back of a fork. Bake until the cookies are golden, 15 to 17 minutes. Let cool 5 minutes on the baking sheets, then transfer to racks to cool completely.