Could anyone tell me where the O2 sensor is located on this model?

My check engine light came on and I took it to Auto Zone. The code was P1131. I was told it was probably an O2 sensor that is affecting my fuel injection system. Awesome. I had an O2 sensor go out once on my Ford Ranger and my dad changed it out for me.

If I could locate the O2 sensor on my car, I think we can get off cheaper. It is in front of the catalytic converter or behind it or both? I think there may only be one, but I am not sure.

Any advice will be greatly appreciated.