You need at least the front O2 sensors for things to work properly. The front sensors read exhaust oxygen and the computer adjusts fuel mixture accordingly, the rear sensors look to make sure all the oxygen is used up in the cats. Adding the front sensors is easy, just weld in some threaded bungs, for the rear use the fooler ( resistor ).

With OBD2, you will trip a code, for the moment the car is running in limp mode.