O2 sensor bad?

[87supramart]

I have had a problem with my car running rich, getting bad gas milage, and just not feeling right for about 3 months. I just took off my exhaust manifold to make my turbo flange templete. When i put it back togeather o forgot to hook up my O2 sensor and i didn't notice for 2 days. The funny thing is that while it wasn't hooked up, it ran exactly the same as before, no check engine light or performance difference. I left my car running and kept plugging, and unplugging it but no dif. is this my problem????