Engine not idling smoothly.. why?

Yes. The front O2 sensors are used by the ECU to help determine the proper air/fuel ratio.

I would clean your throttlebody before you buy any new O2 sensors.

Steps to try:

1. Get a fuel injector cleaner and add to your almost full tank).
2. Get a "carb" cleaner spray clean up the linkages that operate your throttle and related links.

The idea is that the fuel injectors maybe "dirty", hence limiting fuel flow. The second thing is that throttle plate may not be closing all the way to the limits. These 2 things will cause the engine computer to incorrectly set the fuel-air mixture settings, hence the engine rough idle. (bad oxygen sensor or bad throttle position sensor aka TPS will also cause this problem .... but I would do this after I tried the cheaper options).

And change your PCV valve. It can cause a poor idle or may at least be contributing to your problem.

JM