You will need a DMM with a duty cycle function. Connect it between pin 3 of that connector and ground with engine off, key on. If it reads a constant multiple of 10% that is your code. You will be able to go to the factory CD and decipher the code.

The most common code will indeed by the O2 sensor, but it may very well not be the sensor itself. There is a connector under the right side front floor mat that often comes loose or needs attention.

Good luck,