Does a questionable catalytic converter alter engine fuel/air mixture?

My 2003 3.0L petrol engine X-Type showed a code P0420, which would indicate a possible problem with the bank 1 catalytic converter. The car has a little over 106k miles on it. It continued to run well, and got good gas mileage for the 125 highway miles driven after the light came on. No obvious loss of power or RPMs. I had a bad #1 coil pack for about 25 to 50 miles from oil pooling around the plugs due to a weeping valve cover oil seal. I changed the plugs, bad coil pack and valve cover oil seal about 150 to 200 miles before the check engine light came on. It was the only code that showed and I cleared it. I have driven about 25 miles since clearing the code, without the light coming back on. I am not interested in changing the cat. converter. I am wondering if the engine fuel mixture and other operating parameters are determined by the O2 sensor before the cat. converter, and as long as the converter is not plugged or obstructed, can the car be driven as normal otherwise??