Originally Posted by OhioYJ

You need to plug the hole if nothing else. They sell plugs for this, like this for example (<--just an example, plenty of choices out there).

Also note without a flash to disable the the O2 sensors or some sort of O2 sensor simulator you will have a check engine light without the O2 sensor. Most people I know that have tried simulators on other modern vehicles haven't had much luck, and end up needing the ECU flash anyways.