Line an 8x8-inch tin with baking paper and set aside.
Break the chocolate into pieces and place it into a heatproof bowl with the condensed milk.
Place the bowl over a saucepan of gently simmering water to make a double boiler. Stir gently until the chocolate has melted. Alternatively, put it in the microwave in very short bursts, stirring each time until melted.
Crush about ¾ of the Oreos into chunks and stir them into your fudge mixture. Working quickly, transfer it to your lined tin and press it down into place.
Roughly crush the rest of the Oreos and press them on top while the fudge is still wet.
Place in the fridge for around 2 hours until firm. Then remove it from the tin and cut into squares.