Put 300g oreo biscuits in a blender and chop. Melt the butter and add to the biscuit crumbs. Mix everything together well.
Put the mixture in a tart pan or flat cake tin and press down well. Place in the refrigerator for 1-2 hours to cool down.
Heat the chocolate and cream while stirring so that the chocolate melts. Spread the chocolate cream on the cooled base.
Roughly break the remaining oreo biscuits and sprinkle over the tart. Chill overnight. Sprinkle with finely chopped pistachios to taste before serving.