In a small saucepan, combine raspberries over medium heat. Cook for 5-7 minutes until broken down. Strain through a sieve and cool.

Crush Oreo cookies in a food processor. Melt butter and mix with cookie crumbs. Press mixture into tart pan and freeze for 15-20 minutes.

Heat heavy cream in a saucepan until simmering. Remove from heat and add dark chocolate chips, stirring until smooth. Blend in raspberry reduction.

Pour ganache into the chilled crust and drizzle remaining raspberry reduction on top.

Gently swirl the raspberry reduction into the ganache for a marbled effect.

Cover tart with plastic wrap and refrigerate for 1-3 hours until set.

Remove from pan, slice into wedges, and garnish with raspberries or powdered sugar before serving.