Where is the O2 sensor on an Oldsmobile alero?

That would depend on the size of the engine as well as if you are looking to replace the upstream sensor or downstream. Upstream simply means its in the exhaust system BEFORE the catalytic converter while downstream means that it is located after the converter. You need to have someone pull the DTC's, diagnostic trouble codes, from the ECM before you start replacing sensors. The code(s) you pull from that should point you in the right direction to which sensor is giving you a SES light or Check Engine light. I just recently pulled code P0420 on a 2000 Alero with the Quad4 engine. The code simply stated that the output on sensor 1 bank 1 was below threshold. I replaced the sensor and now everything is kosher. Some instances arent that lucky. Good luck with whatever you determine the cause is.

Dale