Does the car have a K&N filter and if so has anyone re oiled the filter lately?
I've seen over oiled air filters cause the MAF to give false readings.

Either way if the MAF is not accurately metering of air at intake and the computer uses lookup tables to relate MAF and 02 sensor readings it may name the 02 sensor but be completely wrong.

I suggest this because it sounds like you've allready replaced the 02 sensor.

How did you obtain the code and definition? If you are using a scan tool then it may be trying to identify which bank the sensor is on.

Good luck.