Preheat the oven to 350˚F. Line two rimmed baking sheets with parchment paper.
Add softened butter, brown sugar, and granulated sugar to the bowl of a stand mixer fitted with a paddle attachment. Set to medium speed and mix until light and fluffy, about 3–4 minutes.
Scrape down the sides of the bowl, add the egg and vanilla, and mix for 1–2 minutes.
Add flour, cinnamon, baking soda, and salt. Mix at low speed until no streaks of flour remain, about 30 seconds.
Add oats, cranberries, and white chocolate chips. Mix at low speed until evenly distributed, about 1 minute.
Using a 1.5 tablespoon-size spring-loaded cookie scoop, scoop 12 cookies on each prepared baking sheet about 3 inches apart. Bake until the cookies are golden brown, about 10–12 minutes. Add a couple cranberries on top as soon as they come out of the oven.
Let cool on the pans for 5 minutes, then transfer to a cooling rack until completely cool.