The O2 sensor won't see a rich mixture with a misfire.
A misfire (chamber not firing the mixture) can be caused by several things, right?...
No spark = no fire
No fuel = no fire
Too much fuel = no fire
No air = no fire
Too much air = no fire
Too low compression = no fire
Ignition timing too far off = no fire
Which one of those misfires does the O2 sensor see as a rich mixture?