In a food processor, pulse the Oreo cookies to a crumb mixture. You will probably have to do this in 3-4 batches. Set the crumbs aside in a large bowl.
Melt the white chocolate chips in the microwave in a medium sized microwave safe bowl for about 30-45 seconds or until melted. Stir to make sure all the chips are completely melted.
Pour the condensed milk into the bowl with the chocolate chips and stir until smooth. Pour 1/4 cup of rainbow sprinkles into the mixture and stir again.
Take the chocolate chips/milk/sprinkles mixture and pour it into the bowl with the Oreo crumbs. Stir to completely mix everything.
Pour the mixture into the prepared baking pan. Using a spatula, smooth out the time so it's a smooth layer. Add the remaining sprinkles on top and gently press down with the spatula so they stick to the top.
Cover with foil or plastic wrap and place in the refrigerator for at least 1 hour to chill.
Remove from the refrigerator, cut into small squares and serve. Want to keep for later on? I love them frozen too! Place the min a freezer bag and freeze. Enjoy!
Notes
If you are making shapes by using cutters instead of squares, I would let you sit this at room temperature for about 10 minutes after taking them out of the fridge. Then cut.