Line an 8 X 8-inch baking pan with parchment paper.
Pour the sweetened condensed milk into a large microwave-safe bowl and microwave for 1 minute. Put the chocolate chips in the warmed sweetened condensed milk and stir until melted. Microwave for an additional 30 seconds if the chocolate is not melting well. Stir in all but about ¼ of the chopped Oreos. Do this fast, the fudge starts to set rather quickly.
Pour the chocolate mixture into the baking pan and smooth into an even layer, working quickly before it starts to set. Top with the remaining Oreo pieces, gently pressing them down into the top of the fudge.
Allow the fudge to sit at room temperature until set, about 2 hours. Remove from the pan, cut into bite-size squares, and serve.