98 ML320 O2 sensor
 

Diagnostics tells us the right bank O2 sensor needs to be replaced on a 98 ML320 with 95K. There's no mention of whether it is the front or rear O2 sensor, though I didn't see an O2 sensor after the converter when I was under the vehicle.

Must I use the expensive replacement O2 sensor, or is there a generic sensor that works and is cheaper, like the Mustang 302 sensor for the 95 E320?

The front ones are harder to get to, but in my opinion are more critical than the rear. The fronts are used to actually set the mixture, but the rear are really just there to analyze the operation of the catalytic converters.
They are both heated sensors and have specific length wiring. I don't know of any generics that will work.
Can you post what code you had and the fault code description.
The front ones aren't all that bad to do if the exhaust system is completely cool.

Gilly