Uh, you missed my point entirely. It doesn't matter where you put the O2 sensor. Being that the first 02 sensor is right after the first cat( which many have expressed a desire to replace with a test pipe such as RalliSpec's which to my knowledge does not include a cat),if the cat is removed, the readings from the O2 sensor are obviously going to be different, thus possibly causing a "check engine" light.
In the various disscussions involving fitting a "test pipe" in place of the OEM cat-fitted up pipe, I did not see any mention about the O2 sensor that is monitoring the performance of the first cat that everyone wants to delete.
I wasn't asking because I didn't know where the O2 sensor would be mounted after the OEM pipe was gone. Jeeze man, I did have a few exhaust systems made in the past.