307, 305 or 350?
O2 Sensor won't CAUSE rich very often, rarely is it ever the problem (Delco sensors), if is often just reporting the problem and the ECM isn't responding or isn't able to.
Look for MAP sensor vacuum line disconnected/broken/leaking, bad MAP, bad coolant temp sensor, bad air temp sensor (TBI), bad thermostat, plugged air filter, stuck float (carb), leaking injector (TBI), high fuel pressure, bad pressure regulator (TBI), plugged catalytic/exhaust, exhaust leak prior to O2 sensor making the O2 think it is lean and richening it (doubtful), broken vacuum line to pressure regulator (TBI), canister purge allowing fuel into carb/throttle body. Worse case, bad ECM, or bad wiring to the injector (TBI).
Start there and see what you find.