O2 sensors

Good news and bad news.
Be glad there are only two and not four. The front one anyalzes the gases going into the converter and the rear one analyzes that geses coming out of the converter to make sure its doing it's job. Unless they are faulty, don't touch them. Use the money saved to take your sweetheart out to dinner.

Since your doing the tune up, don't forget to do some carbon cleaning. The throttle plates and intake valves should get hit. Here is one such product that I like, but there are others:

http://www.seafoamsales.com/motorTuneUpTechGas.htm

https://www.2carpros.com/how_to/how_to_tune_up.htm