o2 sensor help please.

I have a 91 dx w/ a b16a.. my cars running boggy and my ecu is flashing 1 red light when I start the car. Everyones telling me it's the o2 sensor. The setup needs 2 to run good? I have a dc sport header so it doesn't have another bung for an o2 sensor. My question is do I need to get the bung cut into the header or somewhere on the down pipe? Also I was wondering which o2 sensor do I need to buy? thanks a lot for the help in advance.

Have a bung welded in I guess.