Most importantly...please share the OBD II DTC code (DTC= diagnostic trouble code)...which is what the purpose of the federal OBD II mandate is all about...it was to bring a "standard" system for all car manufacturers to adher to as far as electronic engine emissions.
You should be able to get a 5 digit DTC code...post that and that will give all of us a direction to look in. As others mentioned, just because you get an 02 sensor code, doesn't necessarily mean that you need to replace your 02 sensors...there could be an issue upstream or downstream of a sensor that is causing it to register a code.
You (or a mechanic) is supposed to use deductive reasoning & diagnostic trouble shooting to determine why the sensor set the fault code. And this is part of the reason why some states like California stopped auto part stores (like Autozone/Pep Boys/etc) from loaning out OBD II readers...customers were reading the codes and throwing money at parts that didn't fix the issue...and since auto part store employees aren't certified mechanics...they stopped this service that most of the 50 states allow.
So if you don't have a code reader, and you don't have any documents that list the code...take your car to Autozone or Advanced Auto, or Pep Boys etc...and read the code (is your SES light still illuminated?).
Again, due to the federal mandate...many OBD II codes are easily figured out because many of them cross vehicle brands. If you have a DTC code, you can google that code to find common symptoms, causes, and solutions.