Transmission ptob only 3rd gear

My Madza 6 only has 3rd gear. I bought it from someone that redid the exhsust system. They did not install an O2 sensor before the converter. I was told that was the problem. It just doesn't sound correct to me. I was told that the transmission was fine. Thr thing is I don't understand why the person left the O2 sensor out. But once they realized they did, why didn't they fix it instead of selling it this way? Any idess?