Preheat oven to 325°F. Crush Oreo cookies in a food processor until fine crumbs form. Add melted butter and pulse until combined.
Press crumb mixture firmly into the bottom and slightly up the sides of a 9-inch tart pan with removable bottom. Bake for 10 minutes, then cool completely on a wire rack.
Heat heavy cream in a small saucepan over medium heat until tiny bubbles form at the edges. Do not boil.
Place chopped chocolate in a heatproof bowl. Pour hot cream over chocolate and let sit undisturbed for 5 minutes.
Add vanilla extract and whisk gently from center outward until smooth and glossy. Pour ganache into cooled crust and spread evenly with an offset spatula.
Let tart set at room temperature for 2-3 hours or refrigerate for 1 hour until firm.
Arrange fresh raspberries in concentric circles on top of ganache. Dust lightly with powdered sugar if desired. Slice and serve.