I had my bad OEM oxygen sensor (post CAT) replaced with a Denso oxygen sensor a few months ago. Prior to the new oxygen sensor, I was getting approximately 160-180 miles on half a tank (mix of city/highway driving). Since the new oxygen sensor I'm getting about 100-110 miles on half a tank.
It should be noted that my old O2 sensor was rusted on to the exhaust and the sensor itself was completely rusted through. I can only assume this means it wasn't working properly, if at all.
With the new sensor, the car has A LOT more power and is a lot more responsive, however it's absolutely destroying my gas mileage.