A faulity O2 sensor is seldom the cause of high NOx, but HC or CO can point to the O2 sensor.

If your car has an EGR system, do a complete functional test. If not figure out how to retard the timing.

Look at the reduction in NOx due to retarding the timing in the following thread and the thread it refers to.

http://www.peachparts.com/shopforum/showthread.php?p=833484#post833484

I could offer a more comprehensive diagnosis if you would post the complete emission test results - all gases including O2 and the type of test/test conditons - ASM? I/M 240?...

Duke