a couple of months back i did a o2 sensor on my 96 tacoma, (same engine).
i did the sensor before the cat. it is very easy to get to. crawl under the truck a couple of days before you plan on doing it and soak it w/ pb blaster. when you are ready to do it give the nuts a couple of LIGHT taps to help break the rust loose. the rest is pretty straight forward. make sure to put new gasket on there as well.
the only tricky part was that the plug for the sensor was on top of he transmission. If you have a manual tranny like i did all you have to do is take off the shifter boot and it is right there. if it is an auto then you have to feel around, use a mirror to see what you are doing.
hope this helps
good luck
