Preheat oven. Preheat your oven to 350°F (175°C) with the rack in the center position. Then, line two baking sheets with parchment paper or lightly spray them with non-stick spray.
Mix wet ingredients. In a large bowl, beat the butter, granulated sugar, brown sugar, vanilla, and egg with an electric mixer on medium speed for about 3-5 minutes until smooth and slightly fluffy.
Combine dry ingredients. Whisk together the flour, baking soda, and salt in a separate bowl.
Combine the wet and dry ingredients. Gradually add the dry ingredients to the butter mixture, blending on low speed, one cup at a time. Mix until just combined. Do not overmix.
Add mix-ins. Stir in the oats, white chocolate chips, and dried cranberries at low speed until they are combined.
Shape cookies. Scoop tablespoon-sized portions of dough and place them on the prepared baking sheets about 2 inches apart. Then, press the cookie dough down slightly to flatten it into discs. (Skip this step if you prefer thicker, rounded cookies.)
Bake. Bake for 10-12 minutes, until the edges are golden brown. Then, cool the cookies on a wire rack.