Set 3 or 4 Oreos aside and place the rest in a ziploc bag. Crush them with a rolling pin or can. You will want some big and small pieces so don't crush them too much. You can also use a knife to give the cookie a rough chop.
In a large saucepan over medium heat, melt the chocolate chips, condensed milk, sugar, butter, and vanilla. Stir to combine and remove from the heat.
Mix in the chopped Oreos.
Pour the mixture into the lined pan. Top with remaining Oreos, chopping them in fourths first. Place in the fridge for 2-4 hours to set before cutting.
Notes
Make sure and chill before eating! Otherwise it will be too difficult to cut neatly.