Your 96 V6 would have 4 oxygen sensors. Two for each bank. The two in front of the cats in the exhaust manifolds (upstream sensors) are the two that are used by the computer as the main source of information for fuel mixture control. The upstream sensors are the ones most likely to need replacing. The two behind the cats (downstream sensors) in the exhaust manifold are part of the cat monitoring system used by the computer to determine the health of the cats.
The upstream sensor on the front bank (bank 2) is reasonably easy to get to. The upstream sensor on the rear bank (bank 1) is hard to get to. Access is by removing the coil pack and working from above.
Both downstrean sensors are reasonably easy to get to, but most likely don't need to be changed.
You should be aware that most of the time you have oxygen sensor codes it is something other than the oxygen sensors causing the problem, especially if it is both sensors. Depending on what you are trying to do, you may need to do more diagnosis. I would try to diagnose it before changing parts.
