O 2 sensors tell the computer how lean or rich the exhaust stream is. So it effects too lean or too rich or just right(simple version). So if the O2 sensors are not working, it can lead to a lean condition which leads to NOX. If you have a Factory service manual and a old style VOM(swing meter style), you can check them out per the manual. If not and you do not have any idea of how many miles(but feel they could be a bunch) I would just replace them, it could also lead to better fuel ecomany.
Looking at your number you gave, it looks like you are running on the lean side of the fuel flow.
So If we look at those numbers, you need to make it run richer. So yes it could be injectors, but it could be the O2 sensors as well.
With similar number, I have seen good results with the O2 sensors. But testing them first before replacing is a wise choice.
But as the other suggest, the injectors or EGR could also effect NOX.