poor running

Yes, so can about 20 other things. Is there a reason you suspect the O2 sensor?

I'd check the fuel pressure first, than check the Mass Air Flow Sensor. Zee

Yes. You may also want to replace spark plugs. My experience: Went for a 500 mile drive in one day. Car worked perfectly. Next day, car was dying after it warmed up; had to drive in first gear just to keep it moving. Replaced spark plugs with MB specified Bosch type (check with MB for updated part number). Works perfectly. Note that the plugs were previously replaced by an independent shop who had put in "High Performance Platinum" type and had about 40,000 miles on them. Stick with the OEM recommendations and replace every 12,000 miles.