Re: NTK O2 sensors.
Quick question. Is it a 146 or 156 we're looking at?
If i remeber from a brief glance some time back, a 146 has a single HEGO pre cat and the 156 has 2 pre cat (one in each manifold branch pair) and one post cat (to check cat is functional).
As there's only 1 sensor being mentioned i'm assuming a 146.
In which case, in addition to what michaeld has suggested, you could have burnt out valve seats, blowing piston rings (the oil coats the HEGO and stops it working or blows the heater). If there's any way you can check the emmisions on the fly - rolling road? i.e. is it a fualt with the fuel metering, (injectors / ecu / MAF etc) the combustion, (spark plugs / valves / ecu etc) or the sensor?
Your ecu could have locked open loop and just be operating off a stored data set...
Sorry, more to check!!