Line a square 20 cm baking pan with foil, extending over the edges of the pan. Butter the foil, set aside.
Stir the chocolate and the condensed milk in a large heavy saucepan over low heat just until the chocolate is melted and the mixture is smooth.
Remove from the heat.
Stir in the oreos and mix well. Spread the mixture evenly into the prepared pan.
Cover and chill for 2 hours or until firm.
Lift the fudge from the pan using the edges of the foil. Peel off the foil and use a knife to cut the fudge into bars.
Store in an airtight container at room temperature.