In a large bowl, crush the Oreo cookies into small chunks. You can do this by hand or with the end of a rolling pin.
Add in the Rice Krispies and 50g (1¾oz) of marshmallows and combine.
Melt the butter in a large saucepan over a medium heat.
Add the remaining marshmallows and salt into the melted butter and continue to stir until the marshmallows are completely melted.
Combine the marshmallow mixture with the Rice Krispies and Oreos in the large bowl and mix together until the Krispies are completely covered.
Pour the mixture into a 20cm (8in) square baking tin and press down to flatten, ensuring that all of the corners are filled. Chill in the fridge for at least 30 minutes until you are ready to slice into squares and serve.