Generic vs OEM Bosch O2 sensors
 

This question pertains to an Audi A6, but could just as well apply to a 911. The 97 A6 has four O2 sensors, so the total cost of replacement with OEM is >$500. Some people on the Audiworld BB say you can just use generic Bosch 3 wire sensors and splice them into the Audi connectors. Others claim that you should get the ones with "real" OEM connectors, as the generic ones are of lower quality may cause problems.
When I replaced the O2 sensor on the 911, I did buy the OEM one ($120) even though the generic would have been only $70.

I did that with my previous 944, there's absolutely no difference between the two except that the connector hasn't been previously installed on it. The signal wire (i believe it's the black one) has to be precisely connected (like soldered) so that the resistence is as low as possible through the wire. My dad has a generic 4 wire o2 from bosch for his 944T, so you can get them, i just don't know where he got his.

Using the Bosch parts link that I posted earlier I found two entries for this OS. It mentions that the car uses two sensors...is it SOHC or DOHC and 2 or 4 wheel drive?

4wd, 12v 2.8L. If you are referring to Audi, the 96 and later A6s use two 3 wire OS in front of the cats and two 4 wire OS after the cats. The front ones tend to go first.

I get the equivalent Bosch for $35-40 instead of $140 and add off-the-shelf connectors. No functional difference.