PCM Diagnostics & Tuning - Bad o2 causing misfire?

So I kept getting p0305 and p0430 codes. Took my car in and had cat's replaced. They also told me I had a bad o2 sensor and that is probably why my cat's got clogged. I told them I would fix the o2 myself since it would be cheaper. Well I forgot which o2 they said it was and now i am running rich and misfiring (p0305) like crazy. I pull the plug for #5 and it is soaked, i put a new one in and car seems ok for about a week.. then back to misfire and fouled out plug. Car smells very rich. Is all this because of the bad o2 and how do I figure out which one is bad?

could be, or something else causing the problem, the front O2 are the ones used for fueling , I would probably just get 2 new ones depending on the mileage you have