Line an 8x8-inch pan with parchment paper, leaving an overhang for easy removal.
In a medium saucepan, combine chocolate chips, sweetened condensed milk, and butter over low heat. Stir until melted and smooth.
Remove from heat and stir in mint extract and green food coloring if desired.
Fold in most of the crushed Oreo cookies, reserving some for topping.
Spread the fudge mixture evenly into the prepared pan.
Sprinkle the remaining Oreo pieces on top and gently press them in.
Refrigerate for at least 2 hours or until firm.
Lift the fudge out of the pan using the parchment overhang and cut into squares.