Plus... it could very well be there is something else wrong, and that the O2 sensors are fine. Depends on the code thrown.
Mine is prone to toss code 1233 often, which indicates the fuelpump has no power. Which happens if you try to start the car when the aftermarket alarm blocks the start by among other things cutting power to the fuelpump. The error is not wrong, it's exactly what's going on, but it's registered as an error, and it will not clear on its own, unless I clear it using the reader.
I'm not sure why the O2 sensors were changed (unless a prior readout already indicated they were the cause of a problem). There might have been other codes, or the fact that the O2 sensors now finally provide a correct reading may trigger another code somewhere else that was masked by the O2 not working properly. No way to know for sure, unless it's read out.