Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Stir in 1 teaspoon vanilla extract.
In a separate bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the rolled oats until evenly distributed.
Drop tablespoon-sized scoops of cookie dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 12-15 minutes or until the edges are golden brown and the centers are set.
Remove from the oven and let the cookies cool completely on a wire rack.
Once cooled, roughly chop the oatmeal cookies into bite-sized chunks.
In a mixing bowl, whisk together the heavy cream, whole milk, granulated sugar, and 1 tablespoon vanilla extract until the sugar is dissolved.
Pour the mixture into an ice cream maker and churn according to the manufacturer’s instructions, usually about 20-25 minutes, until it reaches a soft-serve consistency.
During the last 5 minutes of churning, add the chopped oatmeal cookie chunks to the ice cream to evenly distribute them.
Transfer the ice cream to an airtight container and freeze for at least 4 hours or until firm.
Serve the oatmeal cookie ice cream scooped into bowls or cones and enjoy!