Preheat your oven to 350°F (175°C). In a medium bowl, mix together the all-purpose flour, oats, and brown sugar. Cut in the cold, unsalted butter until the mixture resembles coarse crumbs. Press this mixture firmly into the bottom of a greased baking dish. Bake for 12-15 minutes, until lightly golden, and set aside to cool.

In a large mixing bowl, beat the cream cheese using an electric mixer on medium speed until smooth, about 2 minutes. Gradually add in the sour cream and granulated sugar, mixing until fully incorporated. Add the eggs one at a time, mixing well after each addition.

If using fresh peaches, slice them into thin pieces and toss with lemon juice. If using frozen peaches, allow them to thaw completely and drain excess moisture.

Once the crust has cooled, pour the cheesecake mixture evenly over the top. Layer the peach slices over the cheesecake filling and sprinkle the oat streusel topping over the peaches.

Bake for 35-40 minutes, until the cheesecake filling is set around the edges and slightly jiggly in the center. The streusel topping should be golden brown.

Remove the bars from the oven and allow them to cool completely at room temperature. Cover with plastic wrap and refrigerate for at least 3 hours.