o2 sensor location

Quick question, on a 97 GSR, is the o2 sensor located on the exhaust manifold, or on the cat? thx

If the car is OBDII, which I think it is, then there are 2 sensors, one on the exhaust manifold, and one on the cat.

Does anyone know the thread size for the o2 sensor? I was given a Tsudo header, and it does not have an o2 bung.It is a nice header though, SS with some real nice tig welds.I work at a steel shop, so I could always make a bung for it.