You can buy plugs to fill the holes. Check your with your local exhaust installer, Midas or anyone similar should have them and they're MUCH cheaper than O2 sensors.
Along the same lines....I installed a Bassanni catted x and do not know where the fronts plug into. There are two female ends underneath the car right next to the rear O2 bungs but nothing around the fronts. I had no loose ends when re-installing the wiring harness and have no idea where the fronts plug into. I was told that you dont even need them.....is this true???
All four of my O2 bungs were filled w/ the plugs....