Where is thermostat olds 3800 series 2 engine?

The thermostat on a Series II Oldsmobile 3800 engine is located on the engine block, at the rear of the engine, near the firewall. It's usually a fairly straightforward location once you get a look at the engine, but it's difficult to be more precise without seeing the specific year and model. You'll likely need to look behind the intake manifold and possibly remove some components to access it. A repair manual specific to your vehicle's year and model will provide the best visual guidance.