Where on the GM 350 vortec engine would you find oxygen sensor?

On a GM 350 Vortec engine, the oxygen sensor(s) are typically located in the exhaust manifold, after the catalytic converter(s). There's usually one, sometimes two, depending on the year and specific emissions configuration of the vehicle. You'll find them screwed into the exhaust pipe. They are not located in the engine itself but rather the exhaust system.