A clogged cat is a possibility, especially after a rebuild. I'd test the AFM first. You haven't sprayed the AFM out with any water or cleaners I hope. Also, if you have any error codes present you cannot set the ignition timing... This may account for part of the problematic behavior.
No, a bad O2 sensor will not affect your fuel mixture to this degree. The ECU does not even use the O2 sensor until the vehicle is up to normal operating temps.
Clear any existing codes and see if any new ones are set. Some codes may show immediately if there is a problem with a component such as 24, 31, 41, 51, 52 (these are the more common error codes); A lean condition or rich condition or O2 sensor fault code will not show up until the vehicle is up to operating temp and has been driven. Depending on what the defect is some codes may show up right away or not until later such as AFM related codes like 24 and 31; So if you don't see these right away don't dismiss the AFM as a suspect.
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