Easy ways to crush Oreos include placing them into a Ziplock bag and using a rolling pin to break them up, and putting them into a food processor before using a few pulses to chop them up. The trick is to find a method that retains all of the crumbs.
When using the rolling pin method, make sure that the bag is securely closed and placed on a flat surface so that the pin does not rip any holes as it crushes the Oreos. Use the rolling pin as if rolling out dough, applying pressure as it is rolled over the cookies in order to break them up. If any large chunks remain, lightly hit them with the rolling pin.
If using a food processor in order to break up the Oreos, pay attention to how finely the cookies are being chopped. It only takes a few seconds for the food processor to bring the Oreos to the desired texture, and keeping it on for too long can result in cookie crumbs that are smaller than desired.
Once the Oreos have been reduced to the desired size, pour them out of the food processor or Ziplock bag into a large mixing bowl before adding the remaining ingredients.