If you replaced the battery with a new battery, then you have other issues for sure. Is the CEL for certain the O2 sensor, and not something else? If the O2 sensor, then lets move on to threads that cover SKREEN, or even possibly, Mystery No Start, which is the RCM, or Relay Control Modul. You use the search feature to find these threads about those issues. I am on the road as usual, but maybe someone who is at home can load these threads for you...but, you need to have a new battery, and check to make sure your battery cables are good as well...cars are very sensitive to voltage....