Well o2 sensors do have a limited life span. That span depends on a number of factors, but the average is around 75,000 miles or so. As for the sensor, buying oem just means you want to throw your money around. Best thing to do is find out who makes the part for nissan, bosch is very common, and then buy the part from another store that isn't going to jack the price up. As long as the sensor fits and is made by a reputable company, it is fine. There is ZERO need to use a nissan branded part.
Just yesterday I got a replacement blower fan resistor made by airtex/wells, a known company. The resistor fins are larger than the stock part, means a larger surface area for cooling he resistor. Yet it was cheaper than getting the nissan part, especially from the dealer.