With a zero oxgen result, I wonder how there can be an exhaust leak down stream of the O2 sender upstream maybe as that will fool the O2 sensor to go rich. that richness will result in HC burning with oxygen in the cat, thus no O2 out the exhaust.
what causes NOx formation? High temps! too high a compression by design or carbon build up, to advanced timing. higher octane fuel may help if this the case, as this will reduce heat if the low octane was detonating and your know sensor is not retarding the timing. So check the knock sensor!
too hot a heat of a spark plug can also cause hot spots, making NOx, try a cooler plug if you have a hot plug.
a very lean mixture can cause hot cylinders. clogged injector, intake leak? check in injectors for flow and spray pattern. reading the spark plug or pulling one injector connector off at a time while at idle noting change in idle speed may also help locate which cylinder has an issue.
flammable spray carb cleaner or starting fluid will pin point intake leaks.
good luck