Add the white chocolate chips to one large mixing bowl and the dark chocolate chips to another.
Add the butter and double cream to a medium saucepan. Heat on a medium setting until the butter is completely melted and the cream is just starting to boil.
Pour 180ml of the cream mixture into the bowl with the white chocolate chips, and whisk vigorously until all of the choc chips have melted in.
Pour the remaining 230ml of the cream mixture into the bowl with the dark chocolate chips, and whisk as above.
Add the roughly chopped Oreo pieces to the dark chocolate ganache mixture and stir through.
Once the crust has been chilling in the fridge for 30 minutes, use two spoons to add alternating dollops of the white and dark ganache mixtures on top of the base, until the tin is full.
Use a cocktail stick or skewer to make swirling motions through the filling, creating a marbled effect.
Put the tin back in the fridge for around 2 hours or until the ganache filling is set.