1994 O2 Sensor location

Where is the O2 sensor located? I get intermittent check engine light and my mechanic said it probably was the O2 sensor. I never brought it back in cause it seemed straight forward enough to do it myself. I just haven't had time to look so if anyone knows that'd be helpful.

The engine light illuminates for other problems other than O2 sensors.
Best to have the codes pulled from memory at a reputable shop and either post them here or have the shop repair them.

Throwing parts at symptoms without diagnosis results in a poor batting average.