I've had a CEL for my O2 sensor for a while. There was nothing wrong with it before I put it in the shop. While in the shop my cat converter was stolen (cut from under the car or something), we had it replaced tho. I'm wondering could my rear O2 sensor be missing all together since it could have came off with the converter? I made a small effort to find it myself but couldn't. Pics showing the rear sensor would probably help me the most.

 

it's screwed right into the top of the cat, kinda at an angle.

Again, without the CEL code, we can't help you. You can't solve automotive problems simply by guessing what you think might be wrong. You also cannot assume that just because one person on the internet has a CEL due to an o2 sensor issue, that that is defintiely the cause for your problem too.

There are literally hundreds of issues that the CEL could be warning you about.

 

The shop, or your car insurance, needs to make it right. If it's a 1996 or newer model there were two sensors, with the second one screwed into the cat itself. If it's still missing, there would be wires that don't go anywhere for that second sensor.