I'm doing a tune up on my car and need to replace the o2 sensor, but someone told me that there are two o2 sensors on the car. I was wondering if a.) there are two and b.) if I should replace one of them and where they are located.

Only the front o2 sensor needs to be replaced, the other one simply measures if the cat is still there.

Thanks, and could anyone confirm this part number is correct for a 95' GM# 25312184

Yep that is the one.
http://www.bonnevilleclub.com/techinfo/?article=49

I am trying to change mines too.
Where is the O2 sensor located in a 94 bonneville?

“you will see a large body spark plug looking dooHickey with 4 wires, that will be the O2... “

This is a description of an O2 Sensor even I can visualize.