Step 1: Preheat your oven to 325°F (160°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy lifting.
Step 2: In a medium bowl, combine the finely crushed Oreo cookies and melted butter. Press this mixture evenly into the bottom of the prepared baking pan. Bake for 10 minutes.
Step 3: While the crust bakes, prepare the cheesecake filling. In a large bowl, beat the softened cream cheese and granulated sugar with an electric mixer until smooth and creamy, about 2-3 minutes.
Step 4: Beat in the eggs one at a time, mixing just until combined after each addition. Do not overmix. Stir in the sour cream and vanilla extract until just incorporated.
Step 5: Divide about 1 cup of the cheesecake batter into a separate small bowl. Stir the unsweetened cocoa powder into this smaller portion until well combined.
Step 6: Pour the plain cheesecake batter over the baked crust. Dollop spoonfuls of the chocolate cheesecake batter over the plain batter. Use a knife or skewer to gently swirl the chocolate into the plain batter to create a marble effect.
Step 7: Bake for 45-55 minutes, or until the edges are set and the center jiggles only slightly when gently shaken.
Step 8: Turn off the oven, crack the door open slightly, and let the cheesecake bars cool in the oven for 30 minutes. Remove from the oven and cool completely on a wire rack.
Step 9: Once fully cooled, refrigerate the cheesecake bars for at least 4 hours, or preferably overnight, to allow them to set completely.
Step 10: Before serving, lift the bars out of the pan using the parchment paper overhang. Slice into squares and top generously with fresh sliced strawberries.