Where is the oxygen sensor 1992 GMC?

You didn't state what engine but O2 sensors are located on the exhaust manifolds or headers. They are scewed into threads into the exhaust manifolds with a wire coming out of them and connecting to another electrical connection. Most vehicles have 1 or 2 but some have as many as 4. You probably only have 2 if you have a V8. A 4 cylinder usually only has 1