it isnt the o2 sensor. 100%.
air fuel guage wont tell you shit. definitely not a narrow band one anyway.
unplug the sensor. i guarentee it wont run any differently.

if your car is an s13 and has a ka24de in it, it should be using an s13 maf and ecu. or nothing else would work. not well at least.

get a multimeter, and check the maf.
find the readings and check that.

also, that isnt how an o2 sensor works. it wont tell the computer to cut fuel.
its last in line as a monitor only.