360 Magnum with stock EFI engine codes

Car seems to be running fine. He’s put about 50 miles on it since I cleared codes and the only codes it has now are transmission related. No o2 codes yet.
It could be a little deceiving in that it will seem to run fine in open loop. Once it warms up or after driving approx 10 min it will look for the 02. The code is def there it's just your not picking it up for some reason.