I just recently bought an 01 X from a guy in CA. The previous owner had removed the OEM exhaust system, took out the cats so he could sell them, and rigged up his own system. The 02 sensors were never re-installed, because there were no cats to put them in. The truck ran fine without the sensors, albeit it ran very rich. When I first bought it, I noticed that it was getting terrible gas mileage (5-6 MPG). I took the truck to a local shop, and the guy installed one 02 sensor in the driver side pipe because it just happened to have a bung in it. After having the sensor installed the truck now gets about 10-12 MPG.
Eventually I will get around to ordering new cats, pipe, and a muffler and rebuilding the system properly.