Mitsubishi Eclipse Gst High idle, bad idle... Honestly Please Help?

The oxygen sensor being bad will affect the idle considerably. The ECU (computer) gets the information on how the engine is burning fuel from those sensors. Without them in place, the idle could be erratic, it could stall out, or it could stick itself in a rich loop until the problem is fixed. First replace the o2 sensor, and adjust the idle per the book, then see if the idle is better.