Preheat the oven to 350°F (175°C).
In a large bowl, cream together 1 cup softened unsalted butter, 1 cup packed light brown sugar, and 1/2 cup granulated sugar until light and fluffy.
Beat in 2 large eggs one at a time, then stir in 1 teaspoon vanilla extract.
In a separate bowl, whisk together 1 1/2 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon ground cinnamon, and 1/2 teaspoon salt.
Gradually add the dry ingredients to the wet ingredients and mix until just combined.
Stir in 3 cups old-fashioned rolled oats until evenly distributed.
Press the oatmeal cookie dough evenly into a 12-inch round pizza pan or a rimmed baking sheet to form the crust.
Bake for 12 to 15 minutes or until the edges are golden brown and the center is set.
Remove from oven and allow the cookie crust to cool completely in the pan on a wire rack.
While the crust cools, prepare the cream cheese frosting by beating together 8 ounces softened cream cheese, 1/4 cup softened unsalted butter, 1 cup powdered sugar, and 1 teaspoon vanilla extract until smooth and creamy.
Spread the cream cheese frosting evenly over the cooled oatmeal cookie crust.
Arrange the sliced strawberries, blueberries, kiwi slices, mandarin orange segments, and halved red grapes decoratively on top of the frosting.
Refrigerate the fruit pizza for at least 1 hour before serving to allow the frosting to set.
Slice into wedges and serve chilled.