I have a Audi a6 with an automatic transmission, and it wont shift out of 1st gear. I think its my 02 sensor?

An oxygen sensor is for emissions. Your car has four of them. When an oxygen sensor goes, it will fire off the CEL (check engine light), this will not affect the transmission. I am thinking that it is a speed sensor that is shot, or the control module. You did not specify what year your Audi is. If you replaced the transmission about a year ago, is there a warranty? Also, was the transmission fluid changed when it was installed? If it is 98 or newer, the car has tiptronic, DO NOT MESS WITH IT! Get it to a German car specialist.I have a Audi a6 with an automatic transmission, and it wont shift out of 1st gear. I think its my 02 sensor?I concur with earlier answer, if you've got Tiptronic, use the manual option (if you got it, its the little +- sign next to your D position) and get it to a dealer/german car specialist. I have a 2004 A4T and the transmission's been acting up but luckily I got my dealer to sort it out. Not cheap, but the only real option you have without ruining your A6. Good luck