O2 sensor - have to backprobe with a scope while the engine is running. Tough to do, practically. Might be able to get away with a datalogging OBD-II scanner. They make a number of them, from $70 to couple of thousand. But that will only report back voltage "snap shots" at the polling rate of the OBD-II on the car - something like a couple of Hz. Not enough resolution for a definitive answer - but better than nothing.
If the RPM varies smoothly and the car seem down on power - very possible the upstream (pre-cat O2 sensor) is going bad. Usually lasts about 60K-100K miles tops - sometimes you can get more out of them, sometimes less. Given the model year of your car - even with low mileage, this is a strong possibility.
If you decide to change it - stick with Denso OEM parts. Lots of people have had issues with non-Denso parts on these cars, seems that the car is pretty picky about what electrical and emissions parts you use. I've used Bosch before with a number of issues - that immediately went away when I used a Denso replacement part.
Before you go too far down this path - make sure you get a full tank of good gas in the car. Lot of the symptoms you are describing can be attributed to a bad tank of gas. If the issues still stay after running a tank of known good gas through - then start diagnosing it further.