No offense, but 95_GRANDAM_GT, you cant just callout the O2 for replacement when the system monitors about 10 different sensors (give or take) The ECM is trying to tell you something and you have to extract the codes to diagnose properly. If you start replacing components without knowing what is wrong, you might as well spend the money $$$$$, and replace them all, that is basically what your doing. You can take your car to most parts stores, and they will scan for free in hopes of selling you the defective part. You can make your own determination after you get the codes, and can also post here again to get some help if you need. There is a multitude of things that would make it do this, If its not staying in overdrive (quote"dropping back out"), I would check the tranny fluid level first and service with new filter if it hasnt been done in awhile. TCC could be bad (would set off a code) plugs/wires could be bad (could set off O2 code) EGR could be getting clogged (could set off code) etc. Good Luck!