check engine light came on..? The mechanic says it's the o2 sensor...is it related to the tranny rebuild? Could there have been something he did to mess it up? If it is something he did, I don't want to be charged for it...and I want to go in sounding like I know what I'm talking about
I just put in a new catalytic converter and o2 one year ago...

I'd give your car a little time to adjust to the new transmission. It's got to get all the sensors back in alignment so to speak. Unless you are having noticeable drivibility issues, I'd give it a week. If the light is still on then take it back and see what it says.
Also you might want to make sure that all exhaust parts were sealed back up properly.