Preheat oven to 350°. Line 12 muffin tins with paper liners.
Make crust: In a large resealable plastic bag or a food processor fitted with a metal blade, crush or blend Oreos until fine crumbs form. Transfer to a bowl and pour in melted butter. Stir until crumbs are completely coated and moistened. Press about 1 tablespoon of crumb mixture into the bottom of each liner.
Make cheesecake: In a large bowl using a hand mixer (or in the bowl of a stand mixer using the paddle attachment), beat cream cheese, sugar, flour, cocoa powder, and salt until light and fluffy. Add eggs; beat until well combined. Fold in melted chocolate.
Pour cheesecake filling over crust in prepared liners. Bake cheesecakes until only slightly jiggly in the center, 12 to 14 minutes.
Let cool for 5 minutes in the pan and then transfer to a rack to cool completely. Refrigerate until cold, at least 1 hour.
Make chocolate ganache: Once cheesecakes are cold, add chocolate chips and heavy cream to a microwave-safe dish. Microwave in 10-second intervals until chocolate is melted; stir until smooth and creamy. Spread about 1 tablespoon chocolate sauce on top of each cheesecake. Return to refrigerator to let chocolate sauce set, about 10 more minutes.
Make chocolate whipped cream: In a large bowl, beat heavy cream with chocolate syrup just until stiff peaks form, about 4 minutes. Transfer to a piping bag fitted with a star tip. Top each cheesecake with chocolate whipped cream and garnish with a piece of chocolate.