In a large bowl, use an electric mixer to cream together 1 cup of softened salted butter, 1 ¼ cups packed light brown sugar, and ½ cup granulated sugar. Beat until the mixture is light and fluffy—this will help give your cookies the right texture.
Add 2 large eggs, one at a time, mixing until well combined. Then, stir in 1 ½ teaspoons vanilla extract and 1 tablespoon milk. Continue mixing until fully incorporated.
In a separate bowl, whisk together 1 ½ cups all-purpose flour, 1 teaspoon baking soda, and ½ teaspoon salt. This ensures the leavening agents and seasonings are evenly distributed throughout the dough.
Gradually add the flour mixture to the wet ingredients, stirring just until combined. Be careful not to overmix, as this can make the cookies dense.
Gently fold in 3 cups of old-fashioned rolled oats and 2 cups of chocolate chips. Make sure they are evenly distributed throughout the dough.
Cover the bowl with plastic wrap and refrigerate the dough for at least 1-2 hours, or up to 2 days. Chilling the dough helps the cookies keep their shape while baking.
Preheat your oven to 350°F (175°C). Line your baking sheets with parchment paper or silicone baking mats to prevent the cookies from sticking.
After chilling, use a 4-teaspoon spring-loaded scoop to drop dough onto the prepared baking sheets. Place the dough about 2 inches apart, as the cookies will spread as they bake.
Bake the cookies for about 10 to 12 minutes, or until the edges are lightly browned and the centers are still soft. The cookies will firm up as they cool.
Let the cookies cool on the baking sheets for 5 minutes to allow them to firm up slightly. Then, transfer them to a wire rack to cool completely.